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Multi-purpose 1/0 board for IBM PCs Ever wished you had an "add-on" board so that your PC could directly control external devices such as domestic appliances & motors? Perhaps you want to experiment with process control or robotics. If so, this plug-in board and its accompanying software could be the answer. By GRAHAM DICKER The standard IBM parallel port is an under-used facility on most computers and can provide a terrific resource as an I/O facility. The main problem is that normally you need to breadboard up an interface to' use it. That's where this I/O board comes in. The MAX-386 I/0 board contains a versatile arrangement of facilities as outlined below: • 7 SPDT relay ports, each rated at 2 amps. • 8 switch or pushbutton input lin es, each only requiring a grounding contact: TTL compatible. • 1 digital to analog converter (DAC); 4-bit precision, 0-2.5V • 1 analog to digital converter (ADC); 20-bit precision (if using a 386-based system) or 16-bit (AT) converter. Using single slope conversion. • 4 open collector outputs able to switch 1 amp at voltages up to 60 volts DC; each output reverse protected. • 4 motor drive outputs (up to 40mA each) for low voltage (3V) DC motors. These can be also used for TTL outputs. • 1 programmable timer output (by linking DAC output into ADC input). Maximum output current 200mA<at> 5 volts. What has been done with the MAX386 board (or MAX for short) is to put together a lot of interfaces in a flexible configuration on one PC board. For example, if the DAC facility is required, the relay outputs can be used to control the outside world, or if more switch inputs are required, the relays can be wired up as an input multiplexer (MUX). Also, if more ADC channels are required, the same relays can be wired as an analog MUX. In some applications, more switched outputs are sometimes needed, so some combination logic can be worked out using the relays. The printer port Usually, the IBM PC has three standard addresses available for the printer ports: LPT1=378H, LPT2=278H, LPT3=3BCH. Each of these addresses has three registers, of which not all bits are available. Port availability The multi-purpose 1/0 board features a programmable timer, seven SPOT relay ports, four open collector & four motor drive outputs, & AID & DIA converters. 66 SILICON CHIP Although the MAX board is designed to be used in conjunction with a standard parallel printer port, some ports are not as standard as others. The main differences are that some inputs and outputs for status bits are inverted. Because of this, the software examples given are for the machi nes I have tested as being compatible. To run on non-standard ports, you may have to make small software changes to invert some lines. MAX uses the first register of the parallel port for the seven relays and the select line for reading the input switches. This line also doubles as the trigger line for the Timer/ ADC. · The second register in the set is used to read the multiplexed input lines and the output of the ADC/ Timer. .12v TTL OUT J3 0AC17 01 1N914 0/C OUT J4 5D ~; PORTA 378 J1 0020---"IWr--=+--I C 011 EM401 T ~: ~: ~:: ~:: ~:: RLA7 -12V ~: 0216 4C C 010 EM401' .,. .,. 3B .,. 2A 001 C 08 EM401 .f .,. .,. •5V 0.1J PORTS 379 J1 0315 16 TTL IN J3 4 A1 0413 0512 3 A2 12 2 81 06 10 1 82 0711 8 C1 IC6 74LS157 MUX 220k 13 TIMER OUT J3-1 0 7 C2 6 01 IC5 555 5 02 AOC IN J3-9 0791 "---------------------------+--◄ B EOc VIEWED FROM BELOW ~ EC ,. + 4.7 16VW+ s; 15 .,. 8 .,. MAX 1/0 BOARD Fig.I: the MAX 1/0 board interfaces to the parallel printer port of any IBM or IBM compatible computer and uses it to control relays, motors or TTL inputs. The last port is used to output to the DAC, TTL, Motor and O/C outputs As such, individual control of the last group is not possible, but a single output of a group or an entire group can be controlled appropriately. For example, if you are using the 1 amp O/C switched outputs, you cannot use the DAC but you could use two O/C outputs and two TTL outputs. The relay and input ports, how- 1 SEL A/B ever, can be used concurrently. Now have a look at the circuit diagram of Fig.1. The relay outputs The relay outputs are driven from bits D0-D6 from register 1. A 3.3kQ resistor limits the base current of the 7 BC547 inverter transistors and another 3.3kQ in the collector of these transistors limits the base current of the following 7 BC557 relay driver transistors. 24V relays with a coil resistance of 7400 are used to reduce the total relay current drain. With all seven 24V relays , the total current drain from the combined ±12V rails is only 220mA. Since each relay coil draws only about 30 milliamps, 1N914 signal switching diodes are adequate as back-EMF suppressors to protect the relay driver transistors. The relays used are DPDT types with the contacts paralleled for inMARCH 1991 67 creased current capacity. Each set of relay connections is terminated in a 0.1-inch pitch SIP header (JZ). TTL/switch inputs A total of eight inputs connect (J3) to a 74LS157 quad 2-input multiplexer, IC6. When the logic level at pin 1 is low (O), the 1-inputs (ie, Al, B1, Cl & D1) are selected. When the logic level on pin 1 is high (1) , the 2inputs (ie, A2, B2, CZ & DZ) are selected. The outputs from IC6 are then taken to the register 2 address inputs (379H) . On some computers, these lines are inverted and will need some software to sort this out. The select line to the 74LS157 comes from register 1 address (378H) bit D7. As the inputs are TTL compatible and float high, a grounding switch is all that is required to be detected as an input. Open collector outputs MAX provides four open collector (O/C) outputs and can be provided with a second option of sinking more current by using BD679 Darlington transistors instead of the BD135 devices for Q15, Q16, Q17 & Q18. The BD135 will sink about 1 amp whereas BD679s will sink about 5 amps. A 3.3kQ resistor limits the base drive current to the output transistors, while an EM401 diode provides Where to get the kit A kit of parts for the multi-purpose 1/0 board described here is available from PC Computers, who own the design copyright on this project. · The full kit of parts, including PC board and software disc, is available as an introductory offer to SILICON CHIP readers for $149 plus $10 postage and packing (offer valid until end of June 1991 ). The fully built and tested MAX board costs $269 plus $10 postage and packing . Or you can buy just the MAX PC board, instruction manual and software disc for $39.95 plus $1 O postage and packing. For further information, contact PC Computers, 36 Regent Street, Kensington, SA 5068 . Phone (08) 332 6513 or Fax (08) 364 0902. 68 SILICON CHIP Fig.2: the MAX board is single sided so it uses a number of wire links. Assembly shouldn't take long & you should have it up & running in about an hour or so. the back EMF protection in each case. These devices are driven from the buffered outputs of the 7408/00s, ICs 1, 2, 3 & 4. (Yes, these ICs can be 7408s or 7400s, as we'll explain in a moment). A single IC is used for each bit as some printer ports invert these lines. It is an easy change to replace the 7408 non-inverting device with a 7400 inverting device and hence use standardised software. A separate ground return line is available for the common emitters on J4. By writing applicable software, the O/C outputs can be used to easily drive stepper motors and an example program is provided on the accompanying disc. TTL/motor drive outputs These outputs are individually buffered by the four 7408 devices and, as mentioned above, each one may be interchanged for non-standard printer ports to fix the problem of inverted lines. The outputs of the whole package are paralleled to provide up to 40mA capacity which is adequate to drive PARTS LIST 1 PC board (only available from P.C. Computers) 1 26-way IDC to O825M cable 7 .309-251 24V 750Q coil DPDT relays 14 T0-92 transistor insulators 1 26-way IDC DIP header 1 22-way SIP header 1 18-way SIP header 1 5-way SIP header 1 0.5-metre length of tinned copper wire 1 8-pin DIP socket 4 14-pin DIP sockets 1 16-pin DIP socket Connecting the 1/0 board to your computer is easy - you simply plug it into a vacant expansion slot. Check that nothing shorts against the other boards. small 3V DC motors or normal TTL loads. D-A converter (DAC) The DAC is a 4-bit (16 level) device using a R/2R ladder network on the outputs of the four 7408 devices. In the normal configuration using the 7408 buffers, the DAC output is in inverted mode. The maximum output voltage is approximately 2.5V DC. A-D converter (ADC) The ADC is a unique design in that the usual way computer designers, like IBM and Apple, implement the games port is to use a timer IC like the 555, but always use a variable resistance from the timing junction to ground. This is fine if all you want to use is a variable resistor between specific values for measurements, but the real world measures voltages. The principle behind this ADC is that it uses the pulse width modulation (PWM) of the 555's pin 5 to set the threshhold reference voltage on its internal comparators. The 555 (IC5) is set up as a 1-second monostable and the trigger input (pin 2) is triggered from register O bit D7 going low. This starts the timeout. As the timer is timing out, the software counter counts program loops. The faster the computer the more loops that can be counted and hence (within reason) the higher the resolution. For example, a 45MHz 386 was tested with a compiled version of the sample software (using Microsoft Basic-7) and a count of 2 22 was reached with the input vo ltage at 5 volts, at the end of th e timeout. By comparison, my slow workhorse 4.77MHz XT will only count up to about 1600 using GWBASIC, a resolution of about only 12 bits. Assembly The MAX is built on a single sided fibreglass PC board with the minimum of wire links. A double sided PC board could have been used but the only lines that are picked up from the IBM bus are the power supply rails, +5V and ±12V. As such, the MAX board can be used with any computer provided that the IBM I/O compatibility is preserved, and an external power supply is wired to the 31-way 0.1-inch pitch edge connector. Fig.2 shows the layout diagram.The first step is to insert all the wire links and the resistors which lay flat on the board. The remaining resistors and the two capacitors can now be loaded, with the three DAC 4. 7kQ resistors being mounted upright. The TO-92 (BC5 5 7/BC54 7) transistors can now be loaded, but ensure that the TO-92 spacers are placed under each transistor between the transistor and the PC board. These spacers ensure adequate ventilation, mechanical stability, and that soldering lead temperatures are not exceeded. At this stage , the IC sockets can all be installed, followed by the remaining transistors an d diodes. The assembly can now be completed by soldering in the IDC pin Semiconductors 4 74LS08 quad dual input AND gates (IC1-4; see text) 1 NE555 timer (IC5) 1 74LS157 quad dual input multiplexer (IC6) 7 1N914 diodes (01 -07) 4 EM401 diodes (08-011) 7 BC547 transistors (01-07) 7 BC545 transistors (08-014) 4 80135 transistors (015-018) Capacitors 1 4.7µF 16VW electrolytic 1 0.1 µF metallised polyester Resistors (5%, 1/4W) 1 220kQ 3 4.7kQ 510kQ 183.3kQ headers and the relays.To test the MAX board, first do a visual check to ensure that there are no splats of solder or shorts on the board. Clean off all flux and apply a sealing coat to the bottom of the PC board, making sure that the edge connector is masked off. Plug the MAX board into the expansion bus of your computer and the · DB25-M plug into a parallel printer connector. The test programs supplied with the kit are all written in BASIC and can be used in interpreted or ·compiled versions. All of the programs will display an identification header and ask for the port address location being used. If you don't know, then try them all. No exits are provided from the program so the usual CTRL-C or CTRLBreak is the easiest exit. These programs can also be used as the basis of your own application programs. SC MARCH 1991 69