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This addressable interface card provides eight opto-isolated inputs and eight relay outputs. A flexible interface card for PCs This addressable interface card provides eight opto-isolated inputs and eight relay outputs. It is addressed from the parallel port of your computer and you can have more than one card, each with a different address, hooked to the same port. By RICK WALTERS Got a control application for your computer that can’t be done with your existing hardware? Perhaps you want to design a security system around an older computer. Whatever the reason, there are a host of applications for this input/output card which can be connected to any PC with a standard parallel port. That means it can be used with old PC XTs, 286 or 386 machines which might otherwise be gathering dust. And of course, it can be used with 486 and Pentium machines as well. You could have up to eight of these cards daisy chained and connected to one parallel printer port on a PC. The cards can be set up with an address from 1 to 8, making them individually addressable, or if the need arises, two or more can be coded with the same address in a master-slave setup. Capabilities This interface card is capable of monitoring eight opto-isolated input lines. The inputs to the PC port are normally high and are taken low (via the opto-isolator) when the card input line is grounded. These eight input lines and ground are accessi­ble via a 9-pin “D” female connector. The card can also switch eight relays. Each relay has changeover July 1997  23 PARTS LIST 1 PC board, code 07107971, 177 x 125mm 8 PC-mount relays (RLY1-RLY8) (Altronics S-4160; Jaycar SY4066) 1 D25 PC-mount male connector 1 D25 PC-mount female connector 1 D25 male connector (solder cup or IDC) 1 D9 PC-mount female connector 1 D9 male connector (solder cup or IDC) 1 8 x 2-pin strip 1 shunt for above 1 3-pin terminal block (5.08mm pitch) Semiconductors 1 74HC137 decoder (IC1) 2 74HC573 8-bit latch (IC2,IC3) 1 74HC02 quad NOR gate (IC4) 1 ULN2803 octal buffer (IC5) 8 4N28 opto-isolators (O1-O8) 9 1N914 small signal diodes Capacitors 2 100µF 25VW PC electrolytic 3 0.1µF monolithic ceramic 1 .01µF MKT polyester 2 .001µF MKT polyester Resistors (0.25W, 1%) 2 47kΩ 1 10kΩ 8 4.7kΩ 1 47kΩ SIP (8 resistor 9 pin) Fig.1 (right): the circuit uses a BCD decoder (IC1) and an 8-bit latch (IC2) to control the eight relays. Eight optoisolators are coupled to 8-bit latch IC3 to drive PortB and PortC of the paral­lel connector CON1. confuse you by calling it card 0 we have labelled them cards 1 to 8. If the output which goes low is jumpered to pin 2 of IC4a, and remembering that pin 4 is high, we then have both inputs of IC4a low, which means that its output will be high. This will latch the data present on PortA (pins 2-9 of CON1) into IC2 and output it on pins 12-19. Pins 12 to 19 of IC2 connect directly to IC5, an octal (eight) buffer. This chip accepts 5V logic levels at its input and has open-collector outputs which can switch up to 50V 0.5A loads. Operating the relays (SPDT) contacts and depending on the type used, the contacts may be rated 6A at 240VAC and 12A at 28VDC (Altronics S-4160) or 3A at 12V (Jaycar SY-4066). All relays are accessed via a 25-pin “D” female connector. Note that the PC board tracks and the connector are not rated for 240VAC and they would not carry this current but if high currents were to be switched, connec­tions could be made straight to the relay pins. These features make the card suitable for a wide range of interface applications. capacitor. Thus if pin 4 of IC4b goes high, the latch pin will immediately go high then return to a low level as the 10kΩ resistor discharges the capacitor. This is how we select the card from the parallel port. The three input lines can only address outputs 0 - 7 so rather than Circuit description The full circuit is shown in Fig.1. IC1 is a 74HC137 latched one-of-eight active-low decoder. This means that any BCD (binary coded decimal) code which is present on the three input lines (A,B,C) will cause the output corresponding to this code to change from its normal level of +5V (high) to 0V (low). The eight outputs are pins 7 and 9 to 15. While the latch enable, pin 4, is high, the outputs will change in sympathy with the input code but once the latch pin is low, the input code is stored or latched. This allows us to select one of the outputs, latch it, then alter the code on the input lines while that output stays selected. To this end, the latch enable pin is AC-coupled through the .001µF 24  Silicon Chip The I/O Card is addressed from the parallel port of your computer and you can have more than one card, each with a different address, hooked to the same port. Note that this is the prototype card and some changes have been made, as can be seen from the component overlay diagram of Fig.4. The current artwork uses a 16pin chip for IC1. OK, so how do we operate the relays? The procedure is to apply the code for the particular relays (see Table 1) to PortA then apply the card select code to PortC, taking pin 1 (which is normally high) low, then back high again. Reading the eight inputs is not quite so straightforward. In the newer PCs, the PortA lines are bidirectional; ie, they can be used for inputs as well as outputs. With the older-style machines they can only be used as output lines. Fortunately, the PortB and PortC lines can be used as inputs but we have to use them both to get our eight lines. Now you will realise why it was necessary to latch the PortC data in IC1. The eight inputs (pins 2-9) of IC3 are pulled high with 47kΩ resistors and each resistor has an opto-isolator output connected between it and ground. If the LED in the opto is lit the transistor will pull the pin low. This will be the case if the corresponding diode is grounded. Unfortunately, computer boffins count from zero, unlike normal human beings who were taught to start from one. This means that diode D1 controls input port D0 and so on. We could label the IC pins D1-D8, but we prefer to draw ICs the way you will find them in the data book, otherwise there could be even more confusion. With the I/O card de-selected, pin 1 of IC4a will be low and therefore pin July 1997  25 Fig.2: the top trace shows the strobe pulse at pin 4 of IC4. The middle trace is the differentiated pulse which is present at pin 4 of IC1. This latches the card address when it is high. The bottom trace is the integrated pulse at pins 8 & 9 of IC4. You can see that the capacitor has not charged up to the 2.5V neces­sary to change the output. Fig.3: these scope waveforms show how the card opto-isolator inputs are read. The strobe goes high and stays high long enough for the input of IC4c to pass through the logic switching threshold. Note that the time constant here is 10 times longer than the latch enable time constant for IC2. The LED test jig is used to verify the opera­tion of the eight relays. Table 1: Port A Code To Energise Relays 1, 5 & 8 BCD Value 128 64 32 16 8 4 2 1 Data Line D7 D6 D5 D4 D3 D2 D1 D0 Relay 1 Relay 2 Relay 3 Relay 4 Relay 5 Relay 6 Relay 7 Relay 8 Total 1 0 0 0 1 0 0 1 128 +16 The value to apply to PORTA is 10010001 binary or 145 decimal. 26  Silicon Chip +1 = 145 10 of IC4c and pin 1 (the output enable) of IC3 will be high. This means that pins 12-19 of IC3 will be floating and therefore at the logic levels on the PortB & PortC pins. When pin 1 of IC3 is taken low the logic levels present on the D (data) inputs are transferred to the Q pins. To read the inputs the first step is to select the card as described previously. Under normal conditions, when we select the card and toggle the relays, the time constant of the integrator at the input of IC4c prevents any change in its output (see the scope waveforms of Fig.2). This time, after selecting the card, we set all the lines of PortB and PortC high, except PortC pin 1, which is held low as we now want to charge the capacitor on pins 8 & 9 of IC4c and take its output low (see the scope waveforms of Fig.3). The 47kΩ resistor and this capacitor form an integrator, at the input of IC4c, which will take 10 times longer to reach the logic switching level than the latch enable network on IC2. Once IC4c changes state, the logic levels at IC3’s inputs will be transferred to its outputs which are connected to PortB and PortC and this binary value can be read by the computer software. D9 ensures that the outputs of IC3 are quickly disabled when PortC pin 1 goes high. In practice, the computer software drives the card and you don’t really Fig.4: the component overlay for the PC board. The card selection jumper is to the right of IC1. For conven­ience, all the input-output connections to the card are made with D connectors. have to think about the circuit machinations in order to be able to use it. Building the card The component overlay of the PC board is shown on Fig.4. You can begin by checking the etch pattern on the PC board with the full size artwork of Fig.6. Fix any faults that might be present before you start installing the parts. Fit and solder the 23 links, followed by the resistors and diodes. Next, insert the ICs along with the resistor network (black dot adjacent to pin 1) noting that IC1, IC2, IC4 and IC5 face one edge of the board and the rest face the other edge. The last group is the capacitors, connectors and relays. Check all the soldering again once you have finished as it is quite easy to miss a row of IC pins or one resistor network pin. Testing it For the big test you will need a 25way D cable to connect your computer printer port to the card and a power supply which can deliver 5V and 12V. Connect the power supply to CON4, check­ing that the +5V on the supply goes to the correct connector terminal. Set the jumper to C1. Connect the male end of the cable to the computer printer socket, the female end to the card and turn on the power supply. Relay test It is possible that several relays will click in, depending on the logic levels on the output of IC2. If nothing is heard don’t despair. Load BASIC into your computer and type in lines 10100 of the program shown in Listing 1. You need not type the comments (lower case); they are only there to explain what each line does. When you run it, each relay should energise in turn, with a 1-second delay between steps. If your relays don’t step at all, first check that you have selected card one. Next, check the voltage at pin 2 of IC4. It should be 0V. If you remove Fig.5: use this circuit to make up a jig for checking relay operation on the board. OUT PORTC,10 from line 50 and run the program again, both pin 2 and pin 3 of IC4 should be low and consequently pin 1 should be high. Fig.5 shows the circuit of a LED array which can be con­nected to a July 1997  27 Fig.6: actual size artwork for the PC board. Check your PC board’s copper tracks for defects before you start assem­bly. LISTING 1 10 CLS: KEY OFF: DEFINT A, B, C ‘define A, B, C as integers 20 PORTA = &H378: REM &H378 is for LPT1. use &H278 for LPT2 30 PORTB = PORTA + 1: PORTC = PORTB + 1 ‘define port addresses 40 OUT PORTA,0: OUT PORTC,10 ‘turn all relays off, take C0 high 50 OUT PORTC,11: OUT PORTC,10 ‘select card one, C0 low then high 60 FOR A = 0 TO 7 ‘relays are coded 1, 2, 4, 8, 16, 32, 64, 128 70 OUT PORTA,2^A ‘select relay 1 to 8 in turn 80 OUT PORTC,11: OUT PORTC,10 ‘select card one, strobe high then low 90 B$ = RIGHT$(TIME$,2): WHILE RIGHT$(TIME$,2) = B$: WEND ‘wait one second 100 NEXT A 110 OUT PORTA,0: OUT PORTC,11 ‘turn all relays off 120 OUT PORTB,120: OUT PORTC,5 ‘take input lines high 130 FOR A = 1 to 200: NEXT ‘delay for IC4c increase value if necessary 140 LIN = 0: B = INP(PORTB): C = INP(PORTC) ‘read port input values 150 IF (B AND 128) THEN BIN = B - 135 ELSE BIN = B + 121 ‘comple­ment bit 8 160 CIN = C AND 14 ‘mask high bits and C0 170 IF (C AND 2) = 0 THEN CIN = CIN + 2 ELSE CIN = CIN - 2 ‘co­mplement bit 2 180 IF (C AND 8) = 0 THEN CIN = CIN + 8 ELSE CIN = CIN - 8 ‘co­mplement bit 8 190 CIN = INT (CIN/2): TIN = 255 - (BIN + CIN) 200 FOR A = 0 TO 7: IF TIN/2^A = 1 THEN LIN = A + 1 ‘find low line 210 NEXT 220 LOCATE 24,20: PRINT “Line”;LIN; ‘print it 230 GOTO 140 ‘loop 28  Silicon Chip 25-way D-connector to check relay operation. This is much more effective than trying to listen for relays clicking. Input test Now add the remaining lines (110230) of Listing 1 and run the program again. After the relays stop sequencing, the number 0 should appear at the bottom of the screen. Ground the cathodes of D1-D8 in turn and the corresponding line number should show on the screen. Pressing the Ctrl and C or Ctrl and Break keys together will get you out of the loop. The fun and games in lines 150-190 compensate for the logic inversion of B7, C1 and C3 on CON1. If you have problems with this section you will have to ground the diode and check that the corresponding data input on IC3 goes low. With pin 1 of IC3 low, the Q output should mimic the D input. If this is working, then check for open circuit tracks between IC3’s Q outputs and the corresponding SC pin on CON1.