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Mini Isolated Serial Link This tiny module (about the size of a postage stamp) provides bidirectional, isolated, full-duplex serial communication. That makes it ideal for when two (or more) boards running from separate supplies need to pass information to each other. It can also carry isolated logic signals. Among its many other uses, it can be used to join two of the Battery Balancers (described in this issue). By Tim Blythman T he High-current Four Battery Balancer project starting on page 21 can handle more than four batteries (or cells) by stacking multiple units. But for that to work, they need to communicate with each other, even though their ground potentials will be quite different; possibly as much as 60V DC apart. To connect their onboard serial links so they can work as a single unit, a serial isolator is needed. This little device uses optoisolators to provide thousands of volts of effective isolation while allowing the serial data to pass through unchanged. Another important use for a device like this is connecting a computer to a device that you’re testing, to prevent any possibility of damage should the device malfunction and feed a high voltage to its serial pins. If you have a single Battery Balancer and wish to monitor or control its operation on a computer, it would be a good idea to use this isolator between the two, for safety. We already published the Zero Risk Serial Link in January 2019 (siliconchip.com.au/Article/11360) for this purpose, but that board includes a power supply for the isolated 68 Silicon Chip device, which often isn’t necessary. That makes the board much larger and more complicated than necessary. In cases where both communicating devices have individual power supplies, this design is a better choice. New design By dispensing with the power circuitry and using six passive SMDs, we’ve managed to squeeze the required circuitry into a PCB that measures just 26.5 x 23.5mm. That’s small enough to be connected inline with your serial link and encased in a short length of large di- These same-size renders of the front (left) and rear (right) of the Isolated Serial Link PCB show just how tiny it is. Whether you use vertical header pins, as shown here, or horizontal, as shown in our photos, is up to you. Incidentally, the renders were taken directly from the new Altium Designer 21, which we reviewed in January (siliconchip.com.au/article/14705). Australia’s electronics magazine ameter heatshrink tubing. Despite this small size, it isn’t hard to build. Fig.1 is the complete circuit diagram. The operation is simple. On the transmitting side, a current loop is formed between the TX pin and the selected supply rail (3.3V or 5V) via one optoisolator LED (OPTO2 for CON1 and OPTO1 for CON2). This is via a 220Ω current-limiting series resistor. So when the TX pin is high, no current flows through the LED, and when it is low, about 10mA (for a 3.3V supply) or 18mA flows. This pulls the RX pin at the opposite end low by activating the Darlington transistor in the other half of the optoisolator. When no current is flowing through the LED, the Darlington is off, so that pin is held high by a 1kΩ pull-up resistor. The configuration is identical for data flowing from CON2’s TX pin to CON1’s RX pin as it is in the other direction. A 100nF bypass capacitor stabilises the voltage across the Darlington on either side. Pin headers CON1 and CON2 are identical, and could be soldered directly to one of the communicating boards (eg, a Battery Balancer) using four of the six pins. siliconchip.com.au Alternatively, all six pins of CON1 can be soldered to a CP2102-based USB-serial module, allowing the combination to plug straight into a computer. Note that only four pins are connected in either case. For the CP2102 module, the 3.3V, RX, TX and GND pins are used. JP1 gives us the flexibility to choose which pin is used for power. If JP1 is set to the 5V position, power is taken from the pin next to GND on CON1. For a CP2102 module, this is the 5V USB supply. However, it corresponds to the 3.3V supply pin on the Battery Balancer; the Battery Balancer’s serial port operates at 3.3V, so that is where we want to connect. For CP2102 modules, you would generally place the jumper in the 3.3V position, which connects to the supply pin marked 3.3V on those modules. Indeed, regardless of whether the GND on either side is at the same potential, the Mini Isolated Serial Link can also be used to provide translation between different signalling levels. To keep the PCB small, we have not added a slot in the PCB to increase the creepage distances, as this would require a larger PCB area to prevent the PCB from breaking when flexed. Thus, the Mini Isolated Serial Link is not suitable for mains voltage isolation. Communication details Practically all TTL serial communications we have seen have the signals idling at a high level. Because we have arranged the optos Fig.1: the circuit is practically the minimum necessary for a pair of 6N138 optoisolators. The 220Ω Ω resistors in series with the opto LEDs limit the LED current while the 1kΩ Ω pull-up resistor holds the output high when the opto is off. The 100nF bypass capacitors are the minimum specified in the 6N138 data sheet. SC Ó siliconchip.com.au Features & specifications • Provides optically isolated bidirectional serial communications • Baud rates up to 57,600 (using 6N138) or 1,000,000 (using 6N137) • Each device can have 3.3V or 5V signal levels (ie, it can act as a level shifter) • Supply current (3.3V): between 0mA (TX & RX high) and 13mA (TX & RX low), average ~6mA • Supply current (5V): between 0mA (TX & RX high) and 23mA (TX & RX low), average ~10mA • Offset voltage: up to 100V DC or 60V AC between GND on either side to only switch on when the input voltage is low, and because the Darlington outputs pull low when active, the signal is not inverted across the device. If we had terminated the TX current loops to GND instead of the supply rail, it would instead act as an inverter. You might have noticed that we’re using a different optoisolator in this project compared to the Zero Risk Serial Link. This option is slightly more spaceefficient for similar speeds. Dual versions of the PC817 devices used for the Zero Risk Serial Link exist, but they are now obsolete, so we had to find an alternative. The footprint used by the 6N138 is also very similar to that used by the 6N137 optoisolator that we used previously in the Flexible Digital Lighting Controller (October-December 2020; siliconchip.com.au/Series/351). The 6N137 is a very fast device (up to 10Mbaud), but requires a 5V supply to meet specifications. In other words, if both sides of your Mini Isolated Serial Link will operate at 5V, you could replace OPTO1 and OPTO2 with 6N137s and work at a much higher speed, up to 1Mbaud or possibly even more. But because we wanted this design to have the flexibility to work with devices using 3.3V signalling levels, as it is very common (and a requirement for use with the Battery Balancer), we are using 6N138 parts instead. The 220Ω resistor value is chosen to work with both the 6N137 (at 5V) and the 6N138 between 3.3V and 5V. The 6N138 has much lower current requirements than the 6N137, so you could increase those values up to around 1kΩ if your transmitter has limited current capacity, or you l l MINI ISOLATED SERIAL LINK Australia’s electronics magazine March 2021  69 Fig.2: as suggested by the circuit diagram’s symmetry, the component layout and PCB traces are also symmetrical if rotated 180° about the centre. Ensure that each opto’s pin 1 faces towards the edge of the PCB. To keep the PCB small, we have put the pin markings on its back. Both the overlay and photo are full size. want to reduce the supply current somewhat. Similarly, the 1kΩ pull-up resistors could be increased in value if the current consumption on the output side is a problem. This will limit the maximum baud rate, though, as the circuit depends on this resistor to pull the output high promptly. Our testing shows that this device will work reliably up to 9600 baud with 3.3kΩ LED series resistors (instead of 220Ω) and 10kΩ pull-up resistors replacing the 1kΩ types. The 3.3kΩ value is the largest possible due to the nominal 0.5mA threshold current needed by the opto LEDs for correct operation; the 10kΩ value could go higher, but at risk of worse interference rejection. Maximum baud rate and could simply be used to pass any low-speed logic signals between two systems, such as an error flag, reset signal or on/off signal. Option The few options for this project revolve around the connections to CON1 (and identical CON2) and the corresponding configuration of JP1 and JP2. For connecting to a CP2102 module, use a 6-way header (pins or socket) to suit the module. In this case, the associated jumper is set in the 3.3V position. While we have shown a pin header and jumper shunt, you could use a short wire link to bridge two pads if you will not change this configuration. For our testing, we fitted the unit with a 6-way female header socket to allow a CP2102 module with a pin header attached to plug in, as that is how a CP2102 module typically comes. But you could reverse that, or just solder the two together using a single pin header. For connection to 4-way header on the Battery Balancer, it’s a case of bridging the 5V pad on JP1 or JP2. This means that the four central pads on that side of the Mini Isolated Serial Link (in the order 5V, GND, TX, RX) are available for connection. These four pins would also be the preferred way of using the Mini Isolated Serial Link with jumper wires or similar, if for no other reason but neatness. You could use a 4-way socket header plugged into a 4-way pin header on the Battery Balancer or even solder it directly to the PCB. We’ve built a few variants to show in the photos, so you can see how some of these options work. Since it is a small and simple project, you can make these selections once the other parts have been fitted. Construction The Mini Isolated Serial Link is built on a 27 x 24mm double-sided PCB coded 24102211. Refer to the PCB overlay diagram, Fig.2, to see where the parts go. If you are using SMD (gullwing) optoisolators, fit these first; otherwise, leave the through-hole variants until last. Like any project using surfacemounted parts, solder flux, tweezers, magnifiers and a fine-tipped iron are handy to have, while solder braid (wick) will help with solder bridges. But this project is simple enough that you might get away without them, as long as your eyesight is good! To fit the SMD optoisolators, align the parts with their pads, noting that pin 1 of each part is at the edge of the PCB; the two parts are rotated 180° relative to each other. Tack one lead to its pad and check that it is correct, especially that you can access the pads on both sides of the optoisolator and that all pins are flat against their pads. If not, melt the solder with the iron and tweak the part until it is aligned and symmetrical. Solder the remaining pins. You can flip the board over and apply more solder through the holes in the pads The 6N138 datasheet indicates rise and fall propagation delays of around 10µs and 1.6µs under typical conditions, setting a hard limit of about 100,000 baud as the bits will start to run into each other. A graph also indicates that the rise delay increases with temperature, which will further skew and distort the data. We did some tests with a CP2102 module plugged into each side of the Mini Isolated Serial Link to see what sort of speeds we could achieve with the specified components. This testing occurred at room temperature, so we would expect the results could be worse at higher temperatures. Testing at 115,200 baud led to data being corrupted about once every 20 bytes. This is not surprising given that propagation delays noted above. At 57,600 baud we didn’t see any This oversize photo show how you errors at all, nor at 38,400 baud. could connect two computers over Note that the Mini Isolated Serial Link can also be used in situations a serial link while providing opto-isolation. Two CP2102s are connected to the where it does not carry serial data. It Mini Isolated Serial Link using female header strips at CON1 and CON2. The jumpers JP1 and JP2 are set to the 3.3V position using blobs of solder. will work at any speed down to DC, 70 Silicon Chip Australia’s electronics magazine siliconchip.com.au Build the world’s most popular D-I-Y computer! ALL-NEW COLOUR 2 Again reproduced oversize for clarity, this shows the Mini Isolated Serial Link with a CP2102 on one side (with blue jumper shunt setting this side to 3.3V) and a four-way header on the other side. The second side has a red jumper shunt fitted to source power from the topmost pin on the four-way header. if you want to be sure they are connected properly. Fit the resistors and capacitors similarly. Check each part against the photos and overlay. Secure each part in the correct place with one pin before soldering the remaining pin. Our photos show large but shiny balls of solder. In this case, as long as there are no bridges, more solder is better than not enough. If you are using through-hole optoisolators, fit them now. Gently bend the leads to allow them to slide into the holes. You may be able to feed the leads into one side, then use the PCB to bend the leads so that the other side can be rotated into place, allowing the leads to spring back and hold the part in Parts list – Isolated Serial Link 1 double-sided PCB coded 24102211, 26.5 x 23.5mm 2 6N138 optoisolators (DIP or gullwing SMD; see text for alternatives) 2 1kW SMD resistors, M3216/1206 imperial size (see text for alternatives) 2 220W SMD resistors, M3216/1206 imperial size (see text for alternatives) 2 100nF 50V X7R SMD ceramic capacitors, M3216/1206 imperial size 2 6-pin headers (CON1,CON2) (see text for other options) 2 3-pin headers with jumper shunts (JP1,JP2) (see text for other options) Jumper wires etc to suit your application siliconchip.com.au Plastic Case Optional See SILICON CHIP July & August 2020 place (or use flat pliers or an IC lead straightening tool before insertion). Check that the pin 1 markers are towards the edge of the PCB then solder one pin. Check that the parts are flat, then solder the remaining pins. If you are using pin headers and jumpers, fit these next. If setting the supply options (JP1, JP2) permanently, use short lengths of tinned copper wire (or component lead off-cuts you might have from another build) and trim the excess after soldering them in place. Finally, fit the headers you need and/or solder the board to another device like a CP2102 module or Battery Balancer as needed. 480MHz, 32-bit processor; 9MB of RAM; 2MB flash memory; 800 x 600 pixel colour display Don’t miss your opportunity to experience Australia’s own worldclass, world-famous single board computer that you build and program yourself, using the world’s easiest programming language – MMBASIC. Learn as you build! Testing and usage The Mini Isolated Serial Link is simple enough that it should just work as long as you exercised care during construction. If you must test it first, use the arrangement shown with two CP2102 modules and open two serial terminal programs on your computer. We find that TeraTerm is a simple but versatile terminal program (and it’s free to boot). There will be more information for use with the Battery Balancer next month on how to connect two Balancers using the Mini Isolated Serial Link. Essentially, once they are connected, they should automatically detect each other and begin communicating so that they act as a single five-to-eightbattery (or cell) balancing unit. Our photos show various other ways of connecting the Mini Isolated Serial Link. Because of the inherent symmetry, you can treat each side of the PCB independently to mix and match what you are connecting to it. SC Australia’s electronics magazine And it’s so easy to build because all the hard work is done for you: the heart of the Colour Maximite II, the Waveshare CPU Module (arrowed) is completely pre-assembled and soldered. YOU SIMPLY CAN’T GO WRONG! Short Form Kit includes: Waveshare CPU module pre-loaded with MMBasic the PCB – with solder mask and screen overlay front & rear panels to suit plastic case shown above and all other components required to build the Does not include plastic instrument case, Colour Maximite 2 CR12xx cell or USB power supply/cable 14000 $ All this for only Plus $10.00 p&p in Aust SILICON CHIP SUBSCRIBERS: $AVE 10%! Subscriber’s price just $126 plus p&p Order now (or more information) at www.siliconchip.com.au/shop/20/5508 March 2021  71