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COMPUTER Universal Parallel Input/Output for PCs Hardware design by D. Aggelos Here's a simple project with Windows control software that allows you to control up to eight relays or other actuators, and read back an equal number of input lines, all by means of the parallel port on your PC. 12V 12V D11 3 K1 RE1 IC2 78L05 R9 820 9 C2 100µ 25V C3 100n C4 100n 11 IC4b 1 IC4c 1 IC4d 1 4 D1 6 R1 22 8 5V 5 K9 1N4001 12V RE2 D2 R2 22 K2 D10 IC4e 10 1 IC4f 1 12 The UPIO software and hardware (a board attached to the PC's parallel port) allows you control up to eight relay outputs using a Windows-style user interface designed for user-friendliness. UPIO also allows you read back logic states on input lines. 13 12V IC4 = 74LS05 12V IC4a 2 1 1 11 K3 RE3 Circuit description The circuit that belongs with the UPIO program is very simple and consists of a few low-cost and easy to find parts. The circuit diagram is given in Figure 1. A tristate buffer type 74HCT241(IC5) arranges all input contact reading. An 8-bit latch type 74HCT574 (IC3) is used to preserve the output state during instruction execution. The third essential component is a power driver type ULN2803 (IC1) which enables output relays Re1-Re8 to be actuated and de-actuated. Because the LPT input port consists of just four bits (one nibble), two subsequent readings are required from the HCT241 to obtain the whole (8-bit wide) input word. Supply power comes from an external mains adaptor with an output of approximately 12 volts DC. An on-board regulator type 78L05 (IC2) provides the HCT and LS ICs with a 5-volt supply rail. Note that the ULN2803 and relays are powered from the unregulated 12-V supply. Obviously, the mains adaptor should be able to supply the necessary current (all relays may be actuated at the same time!). K11 D9 R3 D3 IC3 74HCT574 C1 C1 EN 470µ 25V 1 1N4001 22 12V D4 10 +VS 11 12 13 14 15 16 17 18 R5 22 D5 RE5 R4 22 RE4 K4 K10 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 STROBE D0 D1 D2 D3 D4 D5 D6 D7 9 8 7 6 5 4 3 2 1D 12 13 14 15 16 17 18 19 8 7 6 5 4 3 2 1 I8 I7 I6 I5 I4 I3 I2 I1 O8 O7 O6 12V K5 IC1 O5 O4 ULN O3 2803 O2 O1 VEE 9 12V 1 K6 RE6 5V D6 R6 IC5a EN 1 R10 4x 10k 2 3 4 5 22 K13 18 16 14 12 2 4 6 8 C10 C13 R7 22 12V D7 RE7 K7 12V 4x 22n 1 K8 RE8 5V D8 R8 22 IC5b EN 19 R11 4x 10k 2 3 4 5 K12 5V 9 7 5 3 11 13 15 17 C6 C9 20 C5 100n 14 C15 100n 20 C14 100n IC3 10 IC4 7 IC5 10 IC5 = 74HCT241 4x 22n 002011 - 11 Figure 1. UPIO circuit diagram. 20 Elektor Electronics 4/2000 COMPUTER The PCB The PCB for this project was redesigned to reflect Elektor style and standards from artwork originally supplied by the author. The resulting design is shown in Figure 2. The ready-made board is available through our Readers Services as item 002011-1. A PCB-mount `blue ribbon' 36-way Centronics socket is used at the input to allow easy connection to your PC's printer por