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Cloning a PAL/HAL - Part 8

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Part 1 of this series.

A very common part is the PAL16L8, a 20-pin device (Datasheet). You'll see this part on vintage gear a lot after about 1988.

Like the PAL10L8, there are 10 inputs, but there's a twist. Where the PAL10L8 had 8 dedicated outputs, the PAL16L8 allows 6 of these signals to be programmed as inputs or outputs. Thus, we can have 10 inputs and 8 outputs, 11 inputs and 7 outputs,...16 inputs and 2 outputs. You can see that the PAL16L8 can replace a number of older parts, which was very attractive to manufacturers. Instead of ordering several different parts, one could just order a bucketload of 16L8s and get the volume pricing. You'll see applications where very little of a PAL16L8 is actually used--in quantity, they could be very inexpensive.

Okay, so how do we see which of the 6 programmable pins are outputs or inputs?

Well, suppose we try to "nudge" a pin with just a few milliamps. If it's an input, the voltage on the pin will will track the voltage we use to "nudge" the pin, as an input draws very little current.

On the other hand, if the pin is an output, the voltage on it will move very little, as (check the datasheet) an output pin can sink or source 20 milliamps with no problem.

To implement this scheme, all we need do is add another 74LS393 counter to give us the 16 possible inputs and take the outputs of the 6 uppermost bits of the 16 and use them to drive the programmable pins, each through a 1 Kohm resistor, which will limit the drive current to no more than 4 milliamps.

The revised schematic is attached.

Next up: How to make sense of our reader's outputs, part 9
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Updated June 12th, 2019 at 10:15 PM by Chuck(G)

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