8 Bit RS-232 D/O Card

For you who wants to experiment with the early computer technology 8 bit Data , this project is very useful to start with, and 8 Bit Data Digital I/O still use in Industrial process , such on factories process control system.

Your old computer still can be use for this experiment, IBM PC XT or AT is fine with Processor 80x86.

And with BASIC computer programming Install, or QBASIC programming . Can Also programming by Turbo Pascal language.   The Circuit Consists Of IC CMOS 4040 , IC 74HC573 & 74HC541 and IC ULN2803.
The Parts lists shown at component lists.

The Design of This Digital Output card is based on accessing PC UART as shown at table2. The basic lines for UART communications is TXD and RXD for Transmit Data and Receive Data. The other lines are DTR, RTS, CTS, DCR, DCD and RI , each for input and output in communication 8 Bit Data via Serial Communication.
The Output voltage level on Serial port RS-232 are represent by +12 V for logic "1" and -12 V for logic "0".
The Connector K1 connected to serial port computer and the output through R1-D1 and R2-D2 and R3-D3 for TXD, DTR and RTS. IC6, IC7, IC8 are optocouplers are safe driving by R1-D1 , R2-D2 and R3-D3 for On/Off transistor inside it , when the voltage +12 V and -12V.  The optocoupler will saturate the transistor if the voltage goes to +12 V and goes to cut off the transistor when voltage goes to -12 V.
The RST and CLK drive 12 bit binary counter, and only 8 bit used . The LEN pulse used to hold latch output from IC2. And IC3 drive LED D4-D11 for shown logic state of DO card.

Computer have four terminal Serial Port ( COM1..COM4) as at table 1 , where each serial port , occupied 8 memory map location. as shown table . The Eight Open Collector Output form IC ULN2803A is final stage of this card, and can apply for switching up to 50V with the total output current not more than 500 mA.

The card can powered from regulate power supply from 9-15 V. On the next article I will show you how to program UART PC.

The PCB for DO Card and the component places.(Souces : Elektor Electronic)