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USB/RS-232C By JIM ROWE Want to connect an older test instrument or PC peripheral fitted with a ‘legacy’ serial RS-232C interface to your late-model PC or laptop? That is a real problem with today’s PCs which only provide USB ports. Here is the solution: build this very small USB to RS-232C serial interface. M any readers have test instruments, GPS receiver modules, Rubidium oscillators or PC peripherals which work very well but they can pose a problem when it comes to hooking them up to a current-model desktop or laptop PC. That’s because many older instruments and devices were fitted with what is now known as a ‘legacy’ serial RS-232C interface, while most recent PCs are only provided with USB ports. Yes, we know you can purchase cheap USB to RS-232C interface adaptors. But many of these don’t work very well – or don’t work at all – with older equipment with RS-232C interfaces. Also, many of these gizmos are not compatible with Windows VCP (virtual com port) drivers and need to have a custom driver installed – which is often a problem in itself. Cheap, not nasty! That’s why we’ve come up with this new interface, which can be built up at very low cost. Total cost, including the PCB, should be about $32 or less. And you should be able to assemble it in just a few hours. All of the parts, including the input and output connectors, fit on a PCB which measures only 76 x 46mm. It fits neatly into the smallest jiffy box currently available – the UB5 size, measuring 83 x 54 x 31mm. About the circuit The complete circuit of the interface, shown in Fig.1, 56  Silicon Chip uses just two chips and not much else. At its heart is IC1, a Microchip MCP2200 ‘USB-UART Protocol Converter’ chip, which seems to be rather similar to a PIC18F14K50 micro but is hard-wired to perform USB/serial and serial/USB conversion. On the serial side it’s coupled to the inverters inside a 74HC14 hex Schmitt trigger inverter device (IC2), acting as serial drivers and receivers. Where’s MAX? And no, we have not coupled the MCP2200 to a MAX3232 or MAX3222 multi-channel RS-232C driver/receiver device – to give the circuit ‘full spec’ bipolar RS-232C compatibility. Our first prototype did use that approach but we found that it would not operate reliably with a number of instruments and devices. These turned out to have a serial interface which provided only ‘unipolar’ 0V/+5V signal swings. This was done (a) to save money and (b) because just about all of the serial ports on earlier PCs would interface quite reliably with these signals, even though they were nominally designed to provide and accept bipolar signal swings, ie, signal swings meeting the full RS-232C specification, which specified from -5V to -15V for a mark or ‘1’, and from +5V to +15V for a space or ‘0’. So after quite a bit of testing and experimentation we decided to replace the MAX3222 driver/receiver device with the 74HC14 shown in Fig.1. It effectively goes back to the old unipolar ‘watered down RS-232C’ configuration but we have found it to work reliably with all of the ‘legacy’ siliconchip.com.au INTERFACE serial ports we’ve been able to try it with, including those with ‘true RS-232C’ ports as well as those with the ‘watered down’ approach. We can’t guarantee that it will work reliably with ALL equipment fitted with a ‘true RS-232C’ port, because there may be some gear out there with a finicky RS-232C driver/ receiver chip which won’t recognise unipolar signals. But we suspect these are few and far between, especially these days. It’s also worth noting that while the original ‘full spec’ bipolar signals were designed to give reliable operation over quite long cables (up to at least 15m long), the unipolar 0V/+5V signals of this interface won’t be capable of anything like this. But since this interface is intended mainly to connect ‘legacy’ instruments and equipment to a nearby PC, this shouldn’t be a problem. Returning to the circuit of Fig.1, you can see that four of the inverters inside IC2 are used as drivers, two in parallel for the transmit data (TxD) line and the other two in parallel for the Ready-to-Send (RTS) handshaking line. The remaining two inverters are used as receivers, for the Receive data (RxD) line and the Clear to Send (CTS) handshaking line. So what’s the purpose of the 1kseries resistors in those ‘receive’ signal lines, and also for diodes D1-D4? These components are included to allow the inputs of the ‘receiver’ inverters inside IC2 to handle both true-RS232C bipolar swing signals as well as unipolar signals. The 1k resistors limit the current flow, while the diodes ensure that the inverter inputs are ‘clamped’ to a maximum DC input level of +5.6V and a minimum level of -0.6V. The circuitry around IC1 is quite straightforward. Pins 18 and 19 are the USB data lines and these connect directly to pins 2 & 3 of USB connector CON1. Pins 2 & 3 of IC1 are the input and output pins for its internal clock oscillator which runs at 12MHz as a result of connecting crystal X1 and the 33pF and 15pF capacitors. The oscillator runs at 12MHz because it connects to an internal PLL (phase-locked loop) which effectively multiplies the clock by four, to achieve the 48MHz needed by its USB 2.0 interface engine. Pin 17 of IC1 is its VUSB pin, which needs to be provided with a 470nF bypass capacitor for correct USB enumeration. Pins 5 & 6 are configured in this application to drive LEDs 1 & 2, which blink to indicate activity on the serial RxD and TxD lines. By the way, the MCP2200 is configured from your PC, using a small (freeware) configuration utility. This can be used to configure the MCP2200 in terms of baud rate, data format and so on. We’ll describe this in detail later. No external power is needed for the circuit as it is powered from your PC itself, via the USB cable and pin 1 of connector CON1. Typical current drain varies between about 18mA and 29mA, depending on the data being sent and received; well within the 100mA limit. Construction As you can see from the photos and the overlay diagram of Fig.2, all of the components used in the interface are +5V 10F +5V Rx LED 10k TANT LED1 100nF USB TYPE B CON1 1 4 2 3 18 RST VDD 470 19 D+ GP6/RxLED 14 15 2 X1 12MHz 33pF 1 GP5 9 3 15pF  K 4 D– 8 A GP4 GP3 GP7/TxLED GP0/SSPND IC1 MCP2200 GP2 GP1/USBCFG OSC1 CTS RxD TxD RTS VUSB OSC2 Vss 20 A 100nF Tx LED  LED2 K A A D3 K 14 470 7 K MMC D1 VDD 8 9 CTS 6 5 RxD 1k 6 5 16 K IC2 74HC14 13 1 3 10 11 17 470nF MMC CON2 K D4 2 12 1k DE-9M PLUG D2 1 A A 6 4 11 10 13 12 2 7 TxD 8 9 RTS Vss 3 4 5 7 SC 2014 usb TO RS-232c SERIAL INTERFACE Fig.1: just two ICs and a handful of other components make up the interface. siliconchip.com.au D1–D4: 1N4148 A K K A LEDS April 2014  57 narrow solder-wick (pressed against the pins concerned using the tip of your soldering iron). All of the remaining components are through-hole parts, which can be fixed to the PCB in the usual way. Fit the resistors first, followed by the capacitors, taking care with the polarity of the 10F tantalum, which is the only polarised capacitor. Then fit crystal X1, followed by diodes D1-D4 – using the diagram Fig.2 to guide you regarding their polarity. Next fit IC2, the pins of which can be either soldered directly to the pads under the PCB or plugged into a 14-pin DIL socket soldered into the PCB. Then both CON1 and CON2 can be fitted, noting that each connector is held onto the PCB via a pair of lugs which are soldered to the copper underneath in addition to the actual connection pins. The final components to be added are LED1 and LED2, which are mounted vertically above the PCB with their leads left at almost full length, so the underside of each LED body is about 16mm above the top surface of the PCB. Make sure you fit the green LED in the correct position for LED1 and the red LED in the LED2 position, and also make sure that they are both orientated with their longer anode lead to the right (towards CON2) as shown in Fig.2. It’s housed in a small jiffy box, small enough to fit in the palm of your hand. There’s no battery as it is powered from the USB port that it’s connected to. mounted on the top of a small double-sided PCB coded 07103141 and measuring 76 x 46mm. This has USB connector CON1 at one end and serial connector CON2 at the other. The complete PCB assembly fits snugly into a UB5 jiffy box. It is used upside-down: the PCB is attached to the ‘lid’ of the box (which becomes the base), using four 15mm long M3 machine screws with four 6mm long untapped spacers and four M3 nuts to hold the PCB in place. The two activity LEDs protrude through matching 3mm holes in the ‘base’ of the box, which becomes its top. There is only one SMD component in the project (IC1), which comes in a 20-pin SOIC package. I suggest that you solder this to the top of the PCB before any of the other components, as this makes it easier. You can hold it in position using a pair of spring-loaded, self closing tweezers or similar while you spot-solder diagonally separated pins (like pins 1 and 11, or 10 and 20) to their pads on the PCB. Then the tweezers can be removed to give you clear access while you solder the rest of the pins on each side. If you do create an accidental solder bridge between adjacent pins, it’s usually easy to remove the bridge using 107103141 4130170 USB/SERIAL LA IRES/BSU EINTERFACE CAFRETNI 4CTx 102014 2 C 100nF LED2 470nF 4 IC1 58  Silicon Chip 3 2 470 15pF 100nF X1 + 12MHz TANT 33pF 10F IC2 74HC14 1 1 10k 5 4 MCP2200 1 4148 A A LED1 Rx 1k 4148 TYPE B 2 470 USB 3 20 CON1 1 CON2 9 8 7 6 DE-9M Box drilling Your PCB assembly should now be complete, ready for mounting to the box lid. But first you’ll need to prepare both the box and its lid, by drilling and cutting the various holes shown in Fig.3. There are eight holes in all - four in the lid for mounting the PCB, two in the ‘base’ of the box for the two activity LEDs, and a rectangular hole at each end for access to connectors CON1 and CON2. Assembly After you have drilled and cut all of the holes and finally removed any burrs, you should be ready to mount the PCB assembly inside the lid. This involves passing the four 15mm long M3 screws up through the holes in the lid, and fitting each one with an untapped 6mm long spacer. The PCB assembly can then be lowered into position with the ends of the screws passing through the matching holes near each corner of the PCB. Then four shake-proof washers and M3 nuts can be fitted to the screws to hold the PCB in position. If you’d like to give your Interface a ‘front panel’ like the one you can see in our photographs (mainly to identify LED1 and LED2), we have prepared the artwork which can be downloaded from siliconchip.com.au. This can be printed and then laminated in a plastic sleeve for protection Fig.2 (left): the component overlay, with a matching same-size photograph at right. 1k 4148 4148 siliconchip.com.au and finally cut to shape and attached to the outer surface of the base of the box (which becomes the top) using thin double-sided cellulose tape. The box can then be up-ended and lowered down over the PCB-and-lid assembly, orientated so that the end with the longer rectangular cut-out is at the CON2 end of the PCB. Make sure that the two LEDs pass up and just protrude through their matching holes. Then the four small selftapping screws supplied with the UB5 box can be used to fasten the box and lid together, to complete the assembly. You may also want to attach four small adhesive rubber or plastic feet to the lid/underside of the Interface, to ensure that the screw heads don’t scratch any surface it’s placed on. Parts List – USB/RS232C Interface 1 UB5 jiffy box, 83 x 54 x 31mm 1 PCB code 07103141, 76 x 46mm 1 12.00MHz crystal, HL-49U/US (X1) 1 USB type B connector, PC-mounting (CON1) 1 DE-9 male connector, PC-mounting (CON2) 4 15mm long M3 machine screws, pan head 4 6mm long untapped spacers 4 M3 nuts with shake-proof washers 4 adhesive rubber or plastic feet (optional) Configuring the MCP2200 Semiconductors 1 MCP2200-I/SO USB 2.0 to UART protocol converter (IC1) 1 74HC14 hex Schmitt trigger inverter (IC2) 4 1N4148 100mA diodes (D1-D4) 1 3mm green LED (LED1) 1 3mm red LED (LED2) While there are no adjustments to be made to the Interface before it can be used, the MCP2200 USB-serial protocol converter chip (IC1) does need to be configured to suit your particular application. As mentioned earlier, this is done by connecting the Interface to one of the USB ports on your PC and then running Microchip’s freeware Configuration Utility. When you first connect the Interface to a USB port on your PC (assumed to be running), Windows will respond by installing its standard ‘virtual COM port’ driver. You can then call up Device Manager (usually via Control Panel) and look under ‘Printers and Devices’ to make sure that you now have a ‘USB serial port’. Otherwise you may need to download and install the Microchip Serial Port Driver from the link mentioned below. Check its Properties to learn which COM port number (LH END OF BOX) Resistors (0.25W 1%) 1 10k (brown black orange brown or brown black black red brown) 2 1k (brown black red brown or brown black black brown brown) 2 470 (yellow violet brown brown or yellow violet black black brown) (RH END OF BOX) (UNDERSIDE OF BOX) 8 B 8.25 15.5 CL 15.5 B CL 8.25 A 13 Capacitors 1 10F 16V tantalum electrolytic 1 470nF 50V multilayer monolithic ceramic (474 or 470n) 2 100nF 50V multilayer monolithic ceramic (104 or 100n) 1 33pF NP0 disc ceramic (33 or 33p) 1 15pF NP0 disc ceramic (15 or 15p) 12 C 6.5 6.5 6 (LID OF BOX) CL B B siliconchip.com.au 25.75 19 25.75 19 Fig.3: drilling detail for the UB5 jiffy box. The rectangular holes at the box ends (for the RS232 and USB sockets) are best made by drilling around the inside of the area with a small (eg, 2mm) drill then enlarging to size with a small file. HOLE A: 31 x 13mm HOLES B: 3mm DIAMETER HOLE C: 12 x 13mm B B ALL DIMENSIONS IN MILLIMETRES CL April 2014  59 Here’s how the PCB mounts on the lid of the box, which becomes the base . . . it has been given, the data format it has set (8 data bits, no parity and one stop bit are usually best) and also check whether Windows is advising that it is ‘working properly’. Set the driver’s baud rate to match that of the instrument/ GPS receiver module/Rubidium oscillator or whatever you’re going to be using the Interface to communicate with. This will probably be either 4800 or 9600baud (bps) but you may need to check in its user manual to make sure. Assuming this first step has gone smoothly, the next step is to download and install Microchip’s custom MCP2200 Configuration Utility. This can be downloaded from their website by typing in this URL: www.microchip.com/ MCP2200. Click on ‘documentation and software’, then scroll down until you find the MCP2200 Configuration Utility. It’s a 5.3MB zipped file. After unzipping, this provides a self-installing version of the MCP2200 Configuration Utility. When you run this then fire up the utility itself, you should see a window like that shown in Fig.4 – although you won’t see any text as yet in the large ‘Output’ box. This box will be blank initially, while some of the smaller boxes may also have different contents. Before you click on the ‘Configure’ button at bottom left, you’ll need to ensure that the contents of all of these smaller boxes are as shown in Fig.4. You probably won’t need to change the contents of the Manufacturer, Product, Vendor ID or Product ID boxes, nor will you need to click on the ‘Update VID/PID’ button. But you may need to click on the check box next to the label ‘Enable Tx/Rx LEDs’, to display the tick as shown. It’s also possible that you may need to click on the check box next to ‘Enable CTS/RTS pins’, if the serial device you’re going to be communicating with needs this kind of handshaking. But this is unlikely with most of the devices you’ll want to communicate with using the Interface. If the Baud Rate: text box is not showing the baud rate you want, click on the down arrow to its right to get the drop-down list box, and then select ‘4800’ or ‘9600’ or whatever baud rate you do need from the list. Then if the I/O Config: text box is showing something other than ‘00000000’, click inside the box so that you can type in the correct ‘00000000’ text string. Similarly if the Output Default: text box is not showing ‘11111111’, click inside that box and type in that text string yourself. Now turn your attention to the LED Function box at lower right, and if necessary click on the ‘Blink LEDs’ radio but60  Silicon Chip ton if this isn’t displaying the ‘selected’ bullet. Similarly click on the ‘100ms’ radio button so that it too is selected. At this stage you should be seeing a display very much like that shown in Fig.4, apart from a blank output window. If this is so, you can now click on the Configure button at lower left. There should then be a brief pause while the Config utility ‘does its thing’ with the MCP2200 chip in your Interface; then the text shown in Fig.4 should appear in the Output window to show that the configuration has been done and your Interface is now communicating with the PC via the USB cable. You can then close the Config utility, because your USB-Serial Interface is now configured and ready for use. What if you decide at a later time that you want to use the same Interface to communicate with a different serial device? That’s not really a problem, because all you’ll need to do is fire up the MCP2200 Configuration Utility again and use it to reconfigure the Interface’s MCP2200 to suit the ‘new’ serial device. You’ll be able to change the baud rate, disable the CTS/RTS pins if handshaking is not needed any more, and so on. A few words about cables That’s about it as far as the Interface itself is concerned, but before closing we had better give some basic information regarding RS-232C serial cables and the ways in which they’re wired. That’s because it’s not easy to buy this type of cable nowadays, so you may need to wire up one or more cables yourself. Another possibility is that you may have one or two older serial cables, but are not sure how they’re wired. This can be frustrating if you try using one to connect between the Interface and a particular device and find they won’t ‘talk Fig.4: Microchip’s Configuration Utility, which can be downloaded free of charge (see URL in text). siliconchip.com.au 3 8 4 9 ‘DTE’ END (PC OR USB INTERFACE) 5 DCD = DATA CARRIER DETECT RxD = RECEIVE DATA TxD = TRANSMIT DATA DTR = DATA TERMINAL READY GND = SYSTEM GROUND DSR = DATA SET READY RTS = READY TO SEND CTS = CLEAR TO SEND RI = RING INDICATOR RxD 7 RTS TxD 8 CTS DTR 9 RI GND ‘DCE’ END (INSTRUMENT, GPS RECEIVER OR RUBIDIUM OSC) A ‘STANDARD’ RS-232C SERIAL CABLE WIRING USING 9-PIN CONNECTORS DE-9F (FEMALE) 1 DCD DSR RxD RTS TxD CTS DTR RI GND 2 3 4 5 DE-9M (MALE) 1 6 2 7 3 8 4 9 ‘DTE’ END (PC OR USB INTERFACE) 5 SOFTWARE MAY NEED THESE PINS LINKED DCD 6 DSR RxD 7 RTS TxD 8 CTS DTR 9 RI GND ‘DCE’ END (INSTRUMENT, GPS RECEIVER OR RUBIDIUM OSC) B ‘BARE MINIMUM’ SERIAL CABLE WIRING USING 9-PIN CONNECTORS DE-9F (FEMALE) (DCD) DSR RxD RTS TxD CTS DTR (RI) GND 1 2 3 4 5 DE-9F (FEMALE) 1 6 2 7 3 8 4 9 ‘DTE1’ END (PC OR USB INTERFACE) 5 NOTE: TxD & RxD CROSS CONNECTED, DTR & DSR CROSS CONNECTED, RTS & CTS CROSS CONNECTED, DCD & RI NOT USED (DCD) 6 DSR RxD 7 RTS TxD 8 CTS DTR 9 (RI) GND ‘DTE2’ END (PC OR USB INTERFACE) C ‘NULL MODEM’ SERIAL CABLE (OR ADAPTOR) USING 9-PIN CONNECTORS Fig. 5: various types of serial cables which may be required for the interface to each other’. First of all, most serial ports on older PCs used DE-9 nine-pin connectors rather than the DB-25 25-pin connectors originally used to interconnect RS-232C serial devices like teleprinters and dial-up modems with minicomputers and mainframes. So you’ll probably only have to concern yourself with cables fitted with a nine-pin connector at each end. The next thing to be aware of is that many ‘RS-232C’ serial devices didn’t use ‘hardware’ handshaking at all. Instead of using any of the handshaking lines of the serial ports and cables, they simply implemented a simple software-driven handshaking protocol, sometimes called “X-on/X-off”). As a result these devices may not even need you to use a nine-conductor cable at all: just a stripped down or ‘bare minimum’ three-wire cable, with only the RxD and TxD data lines plus a ground line. But be warned: even though the device itself may not need any of the handshaking lines, the software running in your PC might need to be ‘tricked’ into thinking that siliconchip.com.au RS-232C SERIAL PERIPHERAL 7 DCD 6 DSR USB / RS-232C SERIAL INTERFACE 4 5 2 TxD 3 1 6 RxD 2 the device is ready for action, by linking together some of the pins at the Interface end of the cable (the RTS and CTS pins, for example). Otherwise the software may regard the device as ‘not present’ or ‘busy’. Right, now take a look at Fig.5, which shows in (a) the way a standard RS-232C serial cable was wired up using 9-pin connectors. You can make up this kind of cable very easily using IDC-type DE-9 connectors and a length of standard IDC ribbon cable, because all of the wires have a ‘straight through’ connection – pin 1 to pin 1, pin 2 to pin 2 and so on. The main thing to remember is that the PC or Interface end of this cable (the so-called ‘DTE’ end, standing for ‘data terminal equipment) has a female (DE-9F) connector, while the other end (the ‘DCE’ or ‘data comms equipment’ end) is usually fitted with a male (DE-9M) connector. This type of cable should be fine for connecting the PC (via the Interface) to many types of ‘legacy’ serial device. But just so you’ll be aware of the options, take a look at Fig.5(b). This shows the wiring of a ‘bare minimum’ threewire cable, which only provides the RxD and TXD data lines plus the ground line. You should be able to use this much simpler type of cable to communicate reliably with many of the ‘legacy’ devices using our new USB-serial Interface – although you may find it necessary to link pins 7 (RTS) and 8 (CTS) of the connector at the PC/Interface end, to keep the software ‘happy’. That’s why the diagram shows the link between these pins in red. Finally, Fig.5(c) shows the wiring for a so-called ‘null modem’ serial cable or adaptor. Quite possibly you won’t need to worry about this type of cable/adaptor, because it was really only used to allow two PCs to be hooked up to each other directly via their serial ports, for exchanging data files etc (although we did need to do this to connect the old Agilent scope shown in the opening photo). As you can see, this type of cable/adaptor has a female DE-9 connector at each end. It also has ‘crossover’ connections linking the RxD and TxD data pins, the DTR and DSR pins and also the RTS and CTS pins - so the ‘outputs’ at each end connect to the ‘inputs’ at the other. A cable wired up this way won’t work if you try to use it to connect your PC and Interface to a ‘legacy’ device like a test instrument, a GPS receiver module or a Rubidium oscillator. You’ll need to either use a different cable or SC rewire it to remove the crossover connections. SILICON CHIP 1 DCD DSR RxD RTS TxD CTS DTR RI GND DE-9M (MALE) USB TO/FROM PC DE-9F (FEMALE) Fig.6: this front panel artwork (which actually attaches to the bottom of the box) can also be downloaded from the SILICON CHIP website (www.siliconchip.com.au). April 2014  61