Cause discovered for failure to test 74182 with chip tester

 INTERACTING WITH THE DESIGNER OF THE RETRO CHIP TESTER PRO

Stephan Slabihoud, the maker of the tester, was kind enough to exchange some emails and lead me to the root cause of the issue, which is the inability of the ATMEL 2560 based I/O pins to sink enough current to pull the inputs of the tested chip properly low. 

His document for the tester mentions excessive sink currents with some TTL logic chips but it also mentioned that he had modified the tester design with smaller protective series resistors to increase the drive current as a solution. I should have taken the hint and examined the data sheet for the chip I was testing, but I rashly assumed 'problem solved' by the modification he introduced into his design.

THE REQUIREMENTS OF THE SN74182N CHIP ARE HIGH

The inputs, four each P for propagate carry and G for generate carry, require from 8ma to almost 15ma of sink current with the design recommendation in the data sheet suggesting that they be driven by a source that could sink up to 20ma. Whoops! Far from the 8ma max available on the tester. 

My failure occurred when the tester was setting the four P inputs low. These require only 8ma max (typically per the datasheet) per pin, so they were easier to drive than the G pins that were currently high for this test but would be blipped low as the testing continued. 

SOLUTIONS TO TEST AS THIS EXCEEDS THE CAPABILITY

In spite of the impressive capabilities of this tester, easily able to test hundreds of logic chip types, a very few older chips are beyond even its capability. In fact, if I were to bridge all the protective resistors of the chip tester I would be putting the sink current dangerously near the maximum of the ATMEL processor chip, possibly damaging it. 

Of course, testing offline using a breadboard is a possibility. I have about a dozen of these lookahead carry chips to test which will be a slow and tedious effort. 

I could skip testing these as the chance I will need to use one is pretty slim. I could postpone the testing until the time when I would actually use one. 

Finally, I can build a small breadboard device with buffers that would drive the chip under test based on the signal injected by the Retro Chip Tester Pro. That is, I would use my adapter that allows me to remove a jumper that bridges any pin between chip-under-test and tester. I could take the pins from the test socket out to my buffer board, with the redriven high current output routed back to the pins applied to inserted chip under test. 

If I do this, I will be able to run through all the tests on the chip in question. Further, if I come across any other TTL chips with excessive drive current requirements I can use the same buffer structure to test them.