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8 Channel D/I Card for RS232

Published in issue 290, July 2000

This project is a card with eight optically isolated TTL-compatible inputs for external connection to the serial port, which communicates through the RS-232 protocol. On this card you will look in vain for a UART or microcontroller chip because the design is based on a 27C256 EPROM.Each digital input is connected to an optocoupler (IC7-IC14) through a resistor (R2-R9), and drives it with a current of 5 mA when the input voltage is 5 Vdc.

Components

Relevant products

Resistors:
R1 = 10kΩ 8-way SIL array
R2-R9 = 1kΩ
R10,R12 = 10kΩ
R11 = 4kΩ7
P1 = 25kΩ preset H
Capacitors:
C1 = 100pF
C2-C5,C11 = 100nF
C6-C10 = 10 µF radial
C12 = 330nF
Semiconductors:
D1-D8 = 1N4148
D9 = 1N4001
IC1 = 4060
IC2 = 4040
IC3 = 27C256 (see text for programming)
IC4 = 74HC151
IC5 = MAX232
IC6 = 7805
IC7-IC14 = 4N28 of CNY17-2
Miscellaneous:
K1-K8 = 2-way PCB terminal block, raster 5mm
K9 = 9-way sub-D socket (female), PCB mount
S1 = 8-way DIP switch

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