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Altera_Forum
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14 years ago

Output Pin Driving Another Output Pin

Hello all, I've run into an unusual problem. I'll start off by describing my goal: I'm designing a circuit that can test for short circuits and open circuits in a wiring harness. The wiring har...
Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
14 years ago

I discussed this with my colleagues at work and they feel there a couple of limiting factors here. One of things that concern me is the fact most wiring harnesses are going to have some large junctions. Lets suppose we have two junctions with upto 20 wires going outward from both of them.

If there is a short circuit between the junctions, the CPLD's pin will need to drive as much as 40 wires. With a 5K pull down resistor, the current from the pin will be about 0.66mA at 3.3V.

If, however, there is a junction of 40 wires the current is about 26.4mA - which exceeds the Max V absolute max. ratings (which aren't continuos anyway!).

Is the answer to this problem reducing the speed of operating and using a larger resistor? A 47K resistor and a 100 wire junction will lead to a current of just 7mA.

OR - should I just use a transistor array IC at the output of the Max V and use those to drive the wires - in essence, a buffer?