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ZERO RISK SERIAL LINK by Tim Blythman Want to communicate with and/or program a micro that’s connected to mains or a high-voltage supply? Hmmmm . . . r-i-s-k-y – not just to the device, but to you as well! Here’s the SAFE way to do it! B ecause small computer boards like the Micromite, Arduino and Raspberry Pi are so flexible, chances are you will eventually find yourself using them to control some mains-powered or high voltage battery-powered circuitry. But there’s always the risk that those higher voltages could find their way back to your computer, doing untold damage – and in the worst case, it could be YOU that suffers the untold damage! This nifty little project allows you to send serial data over an optically isolated link, entirely preventing the dreaded 230V-in-the-USB-socket syndrome. It can be used for programming the project you are working on, or for monitoring and feedback from a finished project to your computer. Either way, it provides total isolation. It can also translate 5V serial signals to 3.3V and vice versa. You can even use it to pass data between the USB ports on two separate computers without having to make an electrical connection between the two, avoiding the possibility of Earth loops or other similar problems. You may have seen our USB Port Protector project in the May 2018 issue (siliconchip.com.au/Article/11065). If so, you’ll understand our motivation for this project (sob!). But this provides even better protection for your PC. It doesn’t try to shunt excessive voltages and currents – it won’t even let them near your computer! We’ve had laptop USB ports fail while plugged into certain Arduino work-in-progress projects which involved mains and battery power. We aren’t exactly sure how it happened, but it appears that some voltages got to certain pins that they were not supposed to. We wish we’d had this Isolated Serial Link then; it’s an expensive lesson to learn! Projects that feature high voltages and high currents always have the potential for damage to delicate components like microcontrollers and even computers. Where possible, it is best to separate the two. This circuit is simple, easy to build and does just that. It’s also useful for situations even where there are no USB ports involved, eg, to allow two microcontrollers to communicate via a serial link, even if they are running from different supplies which may not share a common ground. What does it do, exactly? The Isolated Serial Link provides two optoisolated data lines suitable for fullduplex serial data (ie, simultaneous The isolated serial link is being used to program an Arduino Uno. While the isolated Serial Link can provide power, a USB cable (as shown) is used here. 68 Silicon Chip Australia’s electronics magazine siliconchip.com.au sending and receiving). Typically, these connect to the TX and RX pins of a microcontroller or USB/serial converter, although they could be used to pass just about any logic-level signal with a switching frequency up to a bit over 100kHz. There is also a third isolated data line which can be used to get an Arduino (or similar) board to go into programming mode, so that new firmware data can be sent over the isolated serial link. The board includes circuitry to automatically generate a reset pulse on the target board when required. The board also has an optional small isolated power supply, capable of providing up to about 100mA at 5V. This circuit is based around a 555 timer IC driving a Mosfet, which in turn drives an isolating transformer. This supply can power some (but not all) Arduino boards without the need for a separate power supply. 5V and 3.3V isolated outputs are provided, to suit various situations. Alternatively, you can omit the 555 IC and transformer (and associated components) and instead mount a pre-built 5V isolated DC-DC converter module capable of delivering up to 200mA (at 5V and/or 3.3V). While this module can provide more current, it is a specialised part (compared to the generic parts used in the transformer-based power supply) and its pins are quite close together, so despite its 1kV isolation rating, it cannot physically provide the same degree of isolation that the transformer does. Shown here significantly over size for clarity (actual board size is 74mm square), this version has a transformer-based power supply (in case the target PCB doesn’t have its own supply.) mon; many SILICON CHIP designs use this arrangement. But this makes it very difficult to debug your software since you have no way of getting feedback on what’s happening in the microcontroller while it is powered from the mains; at least, not safely. Why do I need one? Besides the risk that you could have The advantage of having an Isolated an accidental short between the inSerial Link is that it allows bidirec- coming mains Active and a (suppostional digital communications with edly) low-voltage connection, even no electrical path for current to flow something as simple as a mains cord between the two halves of the circuit. or socket with swapped Active and This can be handy if the two sides Neutral wires (which is not uncommight be at different voltage levels, mon!) could create a lethal situation. Making connections whether fixed or changing. But now, with the Isolated Serial To connect the external circuitFor example, let’s say that you have Link, you can safely get serial data ry, the board has two connectors at designed a circuit which uses a mi- from the microcontroller, even if it’s the left and two at the right. All four crocontroller and some other circuit- floating at mains potential, so you can have pin-outs that match the ubiquiry, which is powered from the mains see what it’s doing. tous CP2102-based USB/serial modusing a “transformerless” power supIt isn’t just mains circuits where it’s ule, available from the SILICON CHIP ply with a current-limiting capacitor useful either. Online Shop (siliconchip.com.au/ feeding a rectifier. This is quite comFor example, you might have a miShop/7/3543). crocontroller with its positive rail conOne of the two headnected to the positive ers on the left side usu- Features & specifications terminal of a battery, ally interfaces with for example, to sim• Provides a fully electrically isola ted, bi-directional serial link one of these USB/serial plify monitoring the • Galvanic isolation up to several hund red volts modules to connect to a current drawn from • Baud rates up to 115,200 computer. that battery via a high• 3.3V or 5V signalling at either end This module can be side shunt. • USB/serial interface module can plugged in or permaIf the battery bank be fitted at either end • Powered from 5V (eg, a USB port nently soldered to the is Earthed, you can’t ) board, depending on connect to the micro • Can be built with isolated 5V & 3.3V supplies for the remote end your requirements. in the usual manner, • Two isolated power supply options, either 100mA total or 200mA total On the other side as you will short out siliconchip.com.au of the board, one of the communications headers will accept a second CP2102 USB/serial module while the other can be used to plug in where a CP2102 module would. Alternatively, you can just wire up the RX/TX/GND serial connections using jumper leads. Australia’s electronics magazine January 2019  69 Fig.2: a scope grab showing the operation of the circuit in Fig.1. The yellow trace shows the input signal and the green trace, the output. Note that the output rise time is much shorter than the fall time, which stretches the length of the output pulse. The higher the signal frequency, the more this affects signal integrity. VccB OPTOCOUPLER 3 1 SIGNAL IN 2 SC  NO ELECTRICAL CONNECTION, ONLY BY LIGHT 20 1 9 GndA SIGNAL OUT 4 RL GndB Fig.1: the traditional method of optoisolating a digital signal. When the input signal is high, current flows through the series current-limiting resistor and LED, lighting up the phototransistor, which pulls the output high. But using a resistor to pull the output low when the phototransistor switches off severely limits switching speed, allowing it to handle serial signals up to only 19,200 baud. the batteries (and that’s a big no-no!). But if you connect it via the Isolated Serial Link, that is no longer the case and you can communicate with and re-program that micro as usual. An Isolated Serial Link can even be useful if both devices are nominally at the same potential. If a circuit has more than one ground connection, there is the potential for a ground loop which can cause electrical noise, possibly interfering with the integrity of the serial data or other signals in the circuit. The Isolated Serial Link avoids the introduction of an extra ground connection, thus eliminating the possibility of any ground loops being caused by the serial connection. Isolating high-speed digital signals The usual method of optocoupling a digital signal is to apply the incoming signal to the optoisolator’s internal LED via a current-limiting resistor, then connect the output transistor either as an emitter-follower or as a common-emitter amplifier, with a pullup or pull-down resistor respectively. The common-emitter version of this method is shown in Fig.1. When the input signal goes high, the internal LED switches on and the light it produces causes the output transistor to switch on, connecting the output to VccB and so pulling it high. When the input signal goes low and the LED switches off, resistor RL pulls the output signal line low, to GndB. Because the output transistor is ac70 Silicon Chip tuated by light, clear plastic between it and the LED provides a high degree of electrical insulation while still allowing signals to travel from one side to the other. But there is a problem with this configuration: the output arrangement is not symmetrical – the transistor pulls the output up much faster than the resistor can pull it down. You can use a lower resistor value to speed it up but that increases current consumption and you can only lower it so far before you overload the output transistor. A scope grab of this configuration operating is shown in Fig.2. The input signal is yellow and the output signal is green. You can see how the pulse is stretched due to the slow switch-off time, despite a relatively low resistor value of 220being used (drawing nearly 25mA when the output is high). This signal distortion will prevent the receiving end from decoding the serial data above a particular data rate. The fastest baud rate we could achieve reliably with this arrangement was 19,200 baud. The common-emitter version of this circuit would suffer from the opposite problem, ie, a slow switch-on, resulting in short pulses (“runts”). The outcome is the same: high-speed serial data will not pass through such a link. A better method To solve this without resorting to specialised high-frequency optoisolators, we are using pairs of optocouplers in a totem-pole configuration, as Australia’s electronics magazine shown in Fig.3. One pulls the output high and the other pulls it low. That gives fast, symmetrical drive with a much-reduced supply current. When the input signal is high, the upper LED is forward-biased and so current flows from VccB, through its output transistor and to the output signal line, quickly pulling it up. And when the input signal is low, the bottom LED is forward-biased and so its associated output transistor quickly pulls the output signal line low, to GndB. Fig.4 is a scope grab of this type of circuit in operation and as you can see, the rise and fall times are now essentially symmetrical. While there is a delay of around 5µs, this will not affect serial decoding as the critical logic level thresholds are delayed consistently. The 115,200 baud limit of this type of circuit is because the delay starts to extend into the next bit time, and at 230,400 baud (the next standard baud rate), the bits are just over 4µs wide, meaning the bits overlap and distort. The only case where this delay might be a problem at lower baud rates is if the outgoing data is synchronised with the incoming data, either through system design or perhaps a carrier sense bus arbitration design, where the transmitter is listening in on the receiver to see that it has full control of the bus. But that’s a rare situation. For normal serial communications, the delay doesn’t matter. By the way, while it might appear that there is a risk that the supply rails siliconchip.com.au VccA 1 2 SC  2  Fig.4: a scope grab showing the same signal as Fig.2 but using the coupling circuit shown in Fig.3. While there is a slight delay between the incoming and outgoing waveforms, the rise and fall times are now similar and short, so the signal can be properly decoded by the receiver at the output end. VccB 3 4 3 1 SIGNAL IN 20 1 9 OPTO1 SIGNAL OUT 4 OPTO2 GndA GndB Fig.3: this shows the push-pull digital optoisolator configuration which we’re using instead. It is a symmetrical arrangement of two optoisolators in a totem-pole configuration. When the input signal is high, the upper optocoupler conducts, pulling the output signal high. A low input signal activates the lower optocoupler, pulling the output low. This will pass a serial stream of up to 115,200 baud. could be shorted out if both optocouplers are switched on simultaneously (eg, with an open-circuit input), phototransistor current is limited to around 20mA by the light intensity generated by the LEDs. With the input floating, the current through the phototransistors is around 4mA, which is insignificant. Circuit description The circuit diagram for the Isolated Serial Link is shown in Fig.5. Connections are made to the Isolated Serial Link at one end via either CON1 or CON2 and at the other end, via either CON3 or CON4. We’ll explain the reason for the pairs of connectors later. At the moment, it’s easiest to ignore CON2 and CON4 and just consider the signals and power flow between CON1 and CON3. Both ends are essentially interchangeable except for the power flow; CON1 receives 5V power to operate the circuit, between pins 1 & 2, while CON3 delivers 5V and 3.3V to any connected circuitry, at pins 1 and 6. Power flows across the isolation barrier from left to right either via transformer T1 or isolated DC/DC converter module MOD1, depending on which is fitted. Serial data signals pass in both directions in the manner described earlier, using optocouplers OPTO1OPTO4. Data delivered to pin 3 of CON1 appears on pin 3 of CON3 and data delivered to pin 4 of CON3 appears on pin 4 of CON1. siliconchip.com.au While these connectors can be wired to just about any circuit which uses TTL serial communications, the pinouts are specifically designed to suit the cheap and readily available CP2102 USB/serial bridges. So you can solder or plug such a device at either end of the circuit to provide a USB interface. You can choose whether the serial signals at either end have a 3.3V or 5V swing, to suit the type of device that you’re connecting. This is selected on the CON1 side using JP1, and with JP2 for the CON3 side. Note that when you have one side operating at 3.3V and the other at 5V, the optoisolator drive currents are not the same in both directions but we haven’t found this to be a problem – after all, you’re usually applying the signal to a digital input pin on an IC, which has a very high input impedance. Reset signal for micro programming The fifth optoisolator (OPTO5) is used to pass the DTR flow control signal from CON1 to CON3. This is often used with Arduino boards, to reset the micro and put it into bootloader mode, so that the chip can be reprogrammed without any additional user intervention. The DTR signal from a USB-Serial converter is high when the device is idle and no communication is occurring and goes low for the duration of a transmission. It does not just pulse low when data Australia’s electronics magazine is being transmitted but is usually held low any time an application has the serial port open. On typical Arduino boards, an RC network converts the DTR positive-tonegative transition from its onboard USB interface into a brief reset pulse. But this connection is not “broken out” for use with external serial ports or USB/serial converters. However, the RESET pin connection is available, so if we can generate this reset pulse from DTR, we can provide the same function. That’s precisely what D1 and its associated 10nF capacitor and 220 resistor do. Normally, with DTR high, the 10nF capacitor is discharged and the DTR pin on CON3 (pin 5) is held high by a 10kpull-up resistor. If the DTR pin on CON1 is externally pulled low, this pulse is coupled through the 10nF capacitor and it powers OPTO5’s internal LED. Its associated phototransistor conducts, pulling the DTR pin on CON3 low briefly. So if this is connected to the Arduino (or other micro’s) RESET line, the micro will be reset. C1 charges up quite quickly and so after a short time (around 1-2ms), OPTO5 turns off and the RESET line is released. This is shown in scope grab Fig.6, with the DTR pin of CON1 shown in yellow and the DTR pin of CON3 in green. You can see that both traces drop to 0V around the same time but the green trace returns to a high level shortly afterwards. The reset pin on an Atmega328 miJanuary 2019  71 cro (as used in an Arduino Uno) only needs to be low for 2.5µs to guarantee a reset, so this pulse is more than adequate. Note that this pulse must be shorter than one second for the programming sequence to complete correctly. D1 discharges the 10nF capacitor when the DTR pin of CON1 goes high again. If this function is not required and you want to pass the DTR signal through the isolation barrier unaltered, simply replace the 10nF capacitor with a wire link. Isolated power supply We’ve provided two means of getting power ‘across the gap’. The simplest approach is to use a self-contained, isolated DC/DC converter module (MOD1), which has a 1kV isolation rating. In this case, the components in the blue shaded box at the top of Fig.5 are not needed. The 4.7µF capacitor at left bypasses its input supply while REG1’s 1µF bypass capacitor provides 1 D3 1N5819 T1 3 K IN K 7 2.4k REG1 MCP1700 10 K A 820 D2 1N4148 A some output filtering for the module. REG1 at right is a 3.3V low-dropout regulator which provides a 3.3V rail for any circuitry connected to CON3 or CON4. This is included since many USB/ serial converters also provide a 3.3V supply and it’s useful for powering certain microcontrollers or other circuitry. As an alternative to using this module (eg, if you have trouble obtaining it), we have included the circuitry in the box at the top of Fig.5, which 8 4 VCC RST 3 OUT IC1 THR 555 5 CV 2 TRIG GND DIS A 1 10nF 4.7 F GND D4 1N5819 10 F K ZD1 6 1 F A Q1 IRF1405 G 1 F 5.1V 4 2 D OUT 10 F S 1 NOTE: FIT EITHER MOD1 OR COMPONENTS IN BLUE SHADED BOX 2 3 4 CON5 CON4 GND 5V GND 5V IN IN OUT OUT 1 220 CON1 3.3V IN DTR RX TX GND 5V IN 6 2 5 4 220 2 1 2 TX GND 5V IN CON3 3 6  3.3V OUT 5 220 1 4 5 3 4 3 DTR/RESET 4 RX 3 2 TX  220 2 IN JP2 1 OPTO5 PC817 2 3 JP1 5V 3.3V SIGNAL LEVEL 1 K D1 1N4148 2 A 2 3 IRF1405 SIGNAL LEVEL 3 A K G ZD1 1N4148 A K GND OUT 3.3V 5V 4  1N5819 ISOLATED SERIAL LINK 5V OUT MC P1700 10k 1 GND 1 1 220 2 10nF SC 3.3V OUT 4  OPTO4 PC817 6 1 20 1 9 6 2 CON2 RX (DTR) 3 OPTO3 PC817 3 DTR RX 5 OPTO1 PC817 1 3 4 3.3V IN TX 4 4  GND 3 OPTO2 PC817 1 5V OUT 2 B0505S ISOLATED DC/DC CONVERTER (MOD1) A K D D S 4 PC817 1 2 Fig.5: this circuit of the Isolated Serial Link has the optional isolated power supply at the top (blue box). The alternative isolated DC/DC module which can be used instead is near the centre (grey box). The isolated bidirectional serial data link is provided by OPTO1-OPTO4. OPTO5 couples the DTR signal from left to right. 72 Silicon Chip Australia’s electronics magazine siliconchip.com.au comprises a complete isolated power supply. If these components are fitted, you do not need MOD1. Timer IC1 is configured as an astable oscillator and it drives the gate of Mosfet Q1, which sinks current through the primary of transformer T1 in brief pulses. These induce pulses of current through the secondary winding, which are rectified using schottky diodes D3 and D4 to produce a ~6V DC rail, which is then regulated to 5V by zener diode ZD1. IC1 uses pretty much the traditional method for a 555-based oscillator except that we’ve added diode D2 from pin 7 (discharge) to pins 2 & 6 (trigger/threshold) to reduce its duty cycle below 50%. That’s necessary to limit the output voltage at the secondary of T1 to an appropriate level and also to keep current consumption reasonable (around 100mA). IC1 uses a 10nF timing capacitor which is charged up via the 820 resistor and diode D2 when IC1’s output pin 3 is high. IC1’s off time is determined by the 2.4kresistor and 10nF capacitor, as the discharge pin (pin 7) goes low when output pin 3 is low, so the 820 resistor and D2 do not affect the capacitor discharge rate. The result is a square wave with a high period of around 8µs and a low period of around 15µs, giving a frequency of around 40kHz. Schottky diodes are used to rectify the output of transformer T1 to minimise power loss, as they have a lower forward voltage and faster switching Fig.6: the yellow trace is the signal applied to the DTR pin of CON1 while the green trace shows the signal at the DTR/ RESET pin of CON3. The falling edge of the DTR input generates a 1-2ms low pulse at the reset output, which can be used to reset the microcontroller in an Arduino-compatible board, activating its bootloader and allowing the chip to be reprogrammed. CON1 and CON2 are wired identically. They are both included to provide you with different options for making connections to the board. You would generally fit one or the other but not both. CON1 is near the edge of the board and you can fit a male or female six-pin header which may be vertical or horizontal (eg, using a right-angle type). We recommend fitting a horizontal female header socket to CON1. You can use a vertical socket but bend its leads by 90° before fitting it. It is then possible to plug a CP2102 module fit- ted with a right-angle male pin header into this socket (see below). Or you can plug jumper leads into the header. CON2 is placed further inboard and can be used when mounting a CP2102 USB/serial converter module directly on the board. In this case, you would fit a vertical header and then solder the CP2102 module on top. Turning now to CON3 and CON4 on the opposite side of the board, these are arranged slightly differently. For a start, they are reversed compared to each other but also, the TX and RX pins are reversed between them. So if you fit a male pin header to CON3 (ideally a right-angle type), it has the same pinout as a CP2102 module. So by fitting a CP2102 to CON1/ CON2, the Isolated Serial Link is essentially “transparent” and you can treat it as a simple CP2102 module, but with the added isolation layer. You could also fit some other type The Isolated Serial Link can be used as a “null modem” to allow communication between two computers. Note that two USB/serial converter modules are needed for this application. Since each side is supplied with power, the power transfer circuitry is also unnecessary. ERRATA: When using the Isolated Serial Link for isolating circuitry floating at mains potential, the following precautions must be observed: 1) It must be mounted in an Earthed metal or double-insulated case before connecting it to the mains-powered equipment (ideally, within the same enclosure). Only the isolated connections should be brought outside the case. If mounting in a separate case, the wiring to the mains-powered equipment must be mains-rated and properly insulated at both ends. 2) Either omit the isolated power supply circuitry or build the version using MOD1, not transformer T1. 3) If using MOD1, lengthen the slot underneath it until it nearly touches OPTO1 (the slot is already lengthened on RevH boards). siliconchip.com.au than typical silicon types. The 5V rail at the cathode of ZD1 is not only fed to the 5V output pins of CON3 and CON4 but also to the input of regulator REG1, which as mentioned earlier, supplies the 3.3V output pins on CON3 and CON4. Connector options Australia’s electronics magazine January 2019  73 D3 T1 820 5819 D4 5819 2.4k 10 D2 4148 IC1 555 ZD1 5.1 10 F 10nF 4.7 F 10 F 4.7 F Q1 IRF1405 1 F 1 F OPTO1 220 220 JP1 OPTO2 REG1 SILICON CHIP CON4 220 10nF 220 OPTO4 OPTO5 MCP1700-3.3 +5V GND TXD RXD 220 5V 3.3V CON3 10k 24107181 +5V 220 JP1 3.3V5V 3.3V 5V +3.3V DTR/RST RXD TXD GND +5V 1 F 1 F OPTO1 220 +3.3V JP2 OPTO3 3.3V 5V 3.3V5V CON2 CON1 +3.3V DTR RXD TXD GND +5V MOD1 B0505S MCP1700-3.3 USB to UART +3.3V SERIAL DTR RXD CP2102 TXD GND CONVERTER +5V OPTO2 REG1 CP2102 DTR RXI TXO 10nF 220 OPTO4 OPTO5 GND +5V RXI DTR 3.3V CONVERTER JP2 220 CON2 USB to UART SERIAL TXO SILICON CHIP CON4 OPTO3 3.3V GND 220 5V 3.3V CON3 10k 24107181 +3.3V DTR/RST RXD TXD GND +5V Fig.7: this shows where to fit the components for the version of the Isolated Serial Link which uses a transformer to provide isolated power to circuits connected via CON3 or CON4, drawing power from CON1 or CON2. This shows all four connectors fitted but you don’t have to fit them all – and you can also use different types, to suit your application. Fig.8: if you’re building the version which uses the isolated DC/DC converter module (MOD1) instead of transformer T1 and associated components then you only need to fit the parts shown here. This time we’re showing a CP2102-based USB/serial module mounted on the board via CON2 and another plugged into CON4 but that’s just an example of how you can use it. of header to CON3 and wire it up to another board using jumper leads. Alternatively, you can fit a female socket for CON4 (right-angle preferred), you can then plug a CP2102 module in, potentially giving you a USB socket at both ends of the module. That is why the TX and RX pins are reversed; the two sides can then communicate with each other normally. This is a bit like the old “null modem” cables (remember them?) that allowed two computers to communicate via their serial ports. Note though that if you do fit a CP2102 to the right-hand side of the module, it will provide the 3.3V and 5V supplies, so you should leave out all the power supply circuitry on the Isolated Serial Link board (including both T1 and MOD1) so that they do not try to “fight” each other. Because the DTR/RST signal is not useful in this configuration, it isn’t connected to CON4 at all. It’s up to you whether you want to leave D1 and its associated capacitor and resistor off the board, since they won’t be used. In the absence of a commercial model, we wound our own transformer using a 5A 100µH toroidal inductor. After insulating the winding with tape, we wound on a secondary which matched the number of turns on the “primary”. 74 Silicon Chip Winding the transformer Since we couldn’t find a suitably small transformer for T1, we decided to make one ourselves, starting with a prewound inductor, which forms the primary. The secondary is then wound on top. If you are building the unit with the isolated DC/DC converter module, you can skip to the next section. Start with a 3A or 5A 100µH toroidal inductor (we used Jaycar Cat LF1270). Take a roll of electrical tape and cut it into lengths of approximately 250mm, then cut those in half lengthwise, so you have two thin strips. The completed transformer is held in place on the PCB with a pair of small cable ties through the holes provided. Cut the excess from the ties on the underside of the board. Don’t use wires to hold it in place because they could form shorted turns and seriously degrade performance. Australia’s electronics magazine siliconchip.com.au Parts List – Isolated Serial Link 4.7 F +5V GND OPTO1 220 220 JP1 3.3V5V 3.3V 5V USB to UART +3.3V SERIAL DTR RXD CP2102 TXD GND CONVERTER +5V TXO CP2102 OPTO2 SILICON CHIP CON4 220 CON2 DTR RXI TXO 10nF 220 GND +5V OPTO4 OPTO5 RXI DTR 3.3V CONVERTER JP2 OPTO3 3.3V USB to UART SERIAL 220 5V 3.3V CON3 10k 24107181 +3.3V DTR/RST RXD TXD GND +5V Fig.9: this shows which components you need to install if you’re supplying 5V power to both sides of the board, and do not need an isolated supply to transfer power from CON1/CON2 to CON3/CON4. For example, you would use this configuration if you’re connecting a USB/serial converter module at both ends, as shown here. Wind those strips around the inductor with a slight overlap, forming a complete isolation barrier over the windings, except for two small areas where the leads emerge. Next, cut a 2m length of 0.4mm diameter enamelled copper wire. It’s important to start with the correct length; if it’s too short you won’t have enough wire, and if it’s too long, it will be difficult to wind. If you start with a different inductor, you may need to wind on a different number of turns and will, therefore, need a different length of wire. The number of turns you add should match the number of turns already on the inductor (which will become the primary winding). Starting winding on the opposite side of the core to the existing leads, so that the tails will match up with the pads on the PCB. Leave about 25mm of free wire to connect to the PCB, then wind 50 turns on top of the existing windings, keeping them as tight as possible. The direction of winding is unimportant, as the output is rectified. When finished, cut the remaining wire to match the 25mm initial length, then scrape about 5mm of the enamel off the ends of the two leads and tin them. PCB assembly Fig.7 shows where to fit the components on the PCB for the version using the transformer to pass power across the isolation barrier. If you are building the version that uses the DC/DC converter module, refer to Fig.8 instead. Fig.9 shows how to assemble the PCB if you have 3.3V or 5V DC power available at both ends of the Isolated Serial Link. All three versions are built using the same PCB, which is coded 24107181 and measures 74 x 74mm. The following instructions describe fitting all the parts; ignore the instructions to fit any components which your version does not require. Start by soldering the resistors in place. It’s a good idea siliconchip.com.au 1 double-sided PCB coded 24107181, 74mm x 74mm 2 6-pin female headers (CON1,CON4) [Altronics P5374] 2 6-pin male headers (CON2,CON3) [Altronics P5430, Jaycar HM3212] 2 3-way pin headers with jumper shunts (JP1,JP2) [Altronics P5430 and P5450 or Jaycar HM3212 and HM3240] Capacitors 1 4.7µF 16V electrolytic capacitor 2 1µF MKT or multi-layer ceramic 1 10nF MKT Semiconductors 5 PC817 opto-isolators (OPTO1-OPTO5) [element14] 1 MCP1700-3.3V LDO 3.3V regulator, TO-92 (REG1) 1 1N4148 signal diode (D1) Resistors (all 1% 1/4W metal film) 1 10kW 5 220W resistor Extra parts for version using MOD1 (optional) 1 B0505S-1W 5V-5V DC-DC isolated converter or LME0505SC [element14] or RFM-0505S [Mouser] Extra parts for version using T1 1 100µH 5A toroidal powdered iron inductor (T1) [Jaycar LF1270] 1 2m length of 0.4mm diameter enamelled copper wire (T1) 2 small cable ties 1 NE555 or equivalent timer IC, DIP-8 (IC1) 1 IRF1405 N-Channel Mosfet, TO-220 (Q1) [Jaycar ZT2468, Altronics Z1545] 1 5.1V 1N4733 Zener Diode (ZD1) [Jaycar ZR1403, Altronics Z0614] 1 1N4148 signal diode (D2) 2 1N5819 1A schottky diodes (D3,D4) 2 10µF 16V electrolytic capacitors 1 10nF MKT capacitor 1 2.4kW 1% 1/4W resistor 1 820W 1% 1/4W resistor 1 10W 5% 1/2W resistor 1 500mm length of electrical tape to check each value using a multimeter before fitting them, as the colour bands can be difficult to read. Be sure to trim all the leads neatly after soldering, as stray leads left over could potentially compromise the isolation barrier. Mount the diodes next. D1 and D2 are small 1N4148 types while D3 and D4 are larger schottky diodes. They are all polarised, so check that each cathode band is facing as shown on the relevant overlay diagram before soldering it in place. Note that D3 and D4 face in opposite directions. There is also one zener diode, ZD1, and now is a good time to fit it, with the orientation as shown. The five optoisolators can be mounted next. They are not all orientated the same way. OPTO1, OPTO2 and OPTO5 have their pin 1 facing the top of the board while OPTO3 and OPTO4 have the opposite orientation. Line up the dots and notches on the optoisolators with the PCB and ensure they are sitting flush before soldering all the pins. Australia’s electronics magazine January 2019  75 USB to UART SERIAL 3.3V DTR RXI CP2102 TXO GND CONVERTER +5V D3 T1 2.4k 5819 5819 D4 10 D2 4148 820 Fig.10: here’s how to drive an Arduino board using the Isolated Serial Link, with a CP2102 module to provide the USB/ serial interface. The RST pin connection on the Arduino board allows the board to be placed in bootloader mode, to allow the host computer to program the micro. IC1 555 SC 20 1 9 DC VOLTS INPUT SCL SDA ZD1 5.1 4.7 F Q1 IRF1405 10 F (MOD1 ) 220 JP1 MCP1700-3.3 1 F 1 F OPTO2 REG1 JP2 220 220 OPTO4 OPTO5 +5V SILICON CHIP CON4 220 5V 3.3V CON3 GND 24107181 The MKT and/or ceramic capacitors are next on the list. These are not polarised. Install them where shown, then mount small regulator REG1 with the orientation shown. You will need to bend its leads to suit the PCB pad pattern (eg, using small pliers). Now you can fit the electrolytic capacitors, which are polarised. The longer lead is positive, so feed it into the pad marked with a “+” in each case. The stripe on the can is on the side with the negative lead. IC1 can be soldered directly to the board (preferred) or mounted using a socket. Regardless, the notch in IC1 and the socket should face towards the bottom of the PCB. You may need to straighten the IC legs slightly so that they fit through the holes in the PCB or into the socket. Next, fit the sockets for CON1-CON4. The exact arrangement used will vary depending on how you are planning to use the unit. If you are not sure, fit all the sockets as shown in our photos and on the overlay diagrams and then you have various options later. Figs.7-10 show some examples of various ways to use the board. At the same time, solder the two 3-pin headers for JP1 and JP2 to the board. Solder the primary windings (made with thicker wire) to the pads on the left-hand side of transformer T1 with the thinner secondary connections on the right. Secure the transformer to the board using two cable ties, through the holes in the PCB. If fitting DC/DC converter module MOD1, line up its outline with the footprint marked on the PCB, noting that the leads are closer to one edge than the other. The component markings should face towards the middle of the PCB. Solder it in place, keeping it flat and level. Now mount Q1 with its metal tab facing towards the top of the PCB, as shown. If you like, it can be bent forward to sit parallel to the PCB. In this case, the tab will face up. No heatsink is required. Using it Before plugging it in, install the jumper shunts for JP1 and JP2 to match the voltage of the serial signals that will Silicon Chip IO 12/MISO ARDUINO UNO, UNO , DUINOTECH UNO, FREETRONICS ELEVEN OR COMPATIBLE IO 11/MOSI IO 10/SS IO 9/PWM IO8 GND VIN IO7 IO 6/PWM ADC0 IO 5/PWM ADC1 IO 4/PWM ADC2 10k IO 13/SCK RESET +3.3V OPTO3 10nF GND +5V (B0505S) OPTO1 220 3.3V 5V 3.3V5V CON2 CON1 +3.3V DTR RXD TXD GND +5V AREF 10 F 10nF 5 3 IO 3/PWM 1 IO 2/PWM ADC3 ICSP ADC 4/SDA ADC 5/SCL 76 USB TYPE B MICRO 6 4 2 IO 1/TXD IO 0/RXD be applied to each side of the board. We found the 5V selection to work best for CP2102 USB/serial modules. If in doubt, test the voltage of the TX line of the equipment you are planning to connect while it is powered but not transmitting. Serial data lines usually sit at a high level when idle, so this will give you an accurate reading of the voltage level. Typically, you would connect a computer or other device which can supply power to run the circuit to the left-hand side of the unit (via CON1 or CON2). If you have installed either T1 or MOD1, the unit can supply a modest amount of power to devices connected to either CON3 or CON4, up to around 100mA at 5V. This is enough to power something like a bare Arduino board but it will be overloaded if you try to power a board with a lot of extra accessories such as an LCD screen or motor. In this case, you can power the circuit at the “remote” end using a battery pack, keeping in mind that if you wish to maintain isolation, no part of the two sides should be connected. In this case, you only need to make connections to the following pins on CON3/CON4: RX, TX, GND and RST (if needed). It’s always a bit tricky connecting the TX and RX lines between two boards because there are some cases where you connect the pin labelled TX to TX and other times when you connect TX to RX, depending on the labelling scheme used. So to help remove some of the confusion, we’ve printed small arrows on the PCB (visible in Figs.7-9) which show the direction of data travel on each pin. Treating the unit as an isolated CP2102 board If you have a setup where you would normally use a CP2102 module to communicate with a device but you need isolation, you either plug a CP2102 module into CON1 (female header) or solder it to CON2. CON3 then provides a more-or-less identical function to the original CP2102 pins except for the added isolation layer. So if you have a socket which will accept a CP2102 mod- Australia’s electronics magazine siliconchip.com.au ule header, CON3 will have a matching pin-out and can be used as a direct replacement. Connecting to an Arduino This is especially helpful if your Arduino is connected to circuitry operating at much more than 5V (especially a battery which can supply a lot of current), or even mains. The isolation barrier will prevent any accidental shorts or component failures on the Arduino or any connected modules from damaging your computer. In this case, we suggest you use the board with a CP2102 USB/serial module attached to either CON1 or CON2. Run jumper wires from either CON3 or CON4 to the Arduino board, connected as follows: GND to GND, RX to TX and TX to RX. The reason why TX is not connected to TX and RX to RX is that the signal that is being transmitted by one side is being received by the other. This arrangement is shown in Fig.10. To be able to reprogram the Arduino while it is connected over the Isolated Serial Link, you will also need to connect the pin labelled RST on the Isolated Serial Link to the RST pin on the Arduino. Note that this will only work with Arduino boards that communicate via a USB-Serial IC which is separate to the main processor IC. We have tested this on the Uno and Mega compatible boards but it will not work with boards such as the Leonardo because they do not expose their serial programming lines directly. Boards such as the Nano should allow programming, as siliconchip.com.au they use a similar designto the Uno and Mega, although we have not tested this. Other Arduino variants may or may not work, depending on how they are configured. Note that the power supply built around T1 may be able to supply enough power to the Arduino during programming but it’s possible that it can’t, as Arduino boards can be quite power hungry, even when doing nothing. Using it to connect two computers To provide an optoisolated link between two computers (or a computer and Raspberry Pi), you will need to connect two CP2102 modules to the Isolated Serial Link. Connect one to either CON1 or CON2 and the other to CON4. Since both computers can supply power, none of the power transfer circuitry is needed. Note that the DTR/RST signal will not be used either, so OPTO5 and its associated components could be omitted. Using other USB/serial converters While the board was designed to suit CP2102-based modules, other types can be used. Note though that this unit has been designed to work with TTL level signals, and will not work with RS-232 voltage level signals. Just make sure to set the correct voltages on each side and also connect the correct power and signal connections. Using jumper wires with socket ends onto the pin headers is an easy way to do this. You can even use a minimal amount of cyanoacrylate glue (superglue) to join the socket ends of the jumper wires together, to create a removable harness. SC Australia’s electronics magazine January 2019  77