TL866 Testing TTL 7493 - Page 1
Forum section: electronics / testgear

author: BigO
author_rank: Newbie
timestamp: July 29, 2017, 12:40:05 am
content: Hi.Is there anyone out there with a TL866 who can test a 7493 IC to see if their results differ from mine?I just got a TL866 and tried testing a 7493. It fails on the DIV8 section (controlled by CLKB). I have a pile of 7493's and they all fail, including a couple of brand new ones.I happened to have a 74HC93 and it tested okay so I figured the testing logic in the software was correct.I put an oscilloscope to the clock signals being supplied by the TL866 and found that the signal on pin 1 looks weird. The low voltage is almost 1 volt instead of zero and the high voltage is 4 volts.The clock signal feeding to pin 14 of the chip (pin 40 of the ZIF socket) is 0 to 4 volts, and looks more like what I expected.Thinking the load presented by the chip might be the cause, I built an adapter to switch the two clock signals. Though it still fails the TL866 test (understandably), the logic analyzer shows that the DIV8 section does count up as expected with the other clock pulse applied. The DIV2 section does not count up in this configuration. So, the problem is following the signal coming from Pin 1.Have any of you ever seen similar behavior with a TL866? Can anyone run the same test to see what happens?Given the different behavior between the two pins, I'm thinking the unit is defective. But, if that same issue shows up on other units, maybe the issue is a result of how the software is configuring the pin?It did successfully test several other 7400 series logic chips successfully, but I bought the unit specifically to work on an old arcade game board that has 17 of these counters to test, so I'll probably try to return it if it just can't do this job.Thanks for any assistance or feedback you can provide.

author: amspire
author_rank: Super Contributor
timestamp: July 29, 2017, 04:51:24 am
content: It could be a sick driver on pin 1. The TTL needs 3.2mA to pull that clock pin to low and it could easily be able to manage a CMos chip but not a TTL chip with a sick driver.It could also be there is an error in the test vectors. I have never used a TL866, but many other similar programmers have an edit for the TTL tests and you can view the test vectors. Just as an example, if the clock was low before this test and the next vector made the clock high impedance and then the following vector made it low, an HC chip would probably see that as 0-0-0. A TTL chip would probably see it as 0-1-0.Here are the vectors from a Wellon programmer for that chip:The columns of the vectors apply for pins 1 to 14 sequentially. C means 0-1-0. X means unused.0 and 1 are inputs, L and H are the intended outputs. V=5V supply, G = 0v supply.I do not have a 7493 myself so I cannot test this.V0001 C11XVXXLLGLLXCV0002 000XVXXLLGLHXCV0003 C00XVXXLHGLLXCV0004 000XVXXLHGLHXCV0005 C00XVXXHLGLLXCV0006 000XVXXHLGLHXCV0007 C00XVXXHHGLLXCV0008 000XVXXHHGLHXCV0009 C00XVXXLLGHLXCV0010 000XVXXLLGHHXCV0011 C00XVXXLHGHLXCV0012 000XVXXLHGHHXCV0013 C00XVXXHLGHLXCV0014 000XVXXHLGHHXCV0015 C00XVXXHHGHLXCV0016 000XVXXHHGHHXCA third possibility is that for this IC, CLKB is meant to be connected to pin 12 (QA output). The vectors above simulates this by directly driving the levels of CLKB, but if the TL866 actually tries to connect the two pins with a cmos switch, the cmos switch may have too much resistance to pull CLKB low with a TTL chip. CLKA wouldn't have the same problem as it would be connected to a driver.A test for this would be to deliberately bend out pin 12 so it made no contact to the tester socket and see if pin1 still has a clock.EDIT: Here is a TL866 schematic. If the test vectors are driven directly by the PIC18 processor, only ports D, E and J are rated at 4mA. The other ports are 2mA so it is possible that CLKB is being driven by a port that cannot handle 3.4mA. CLKA may be driven by one of the higher power ports.http://www.minipro-ua.com/_fr/0/__TL866.pdf

author: BigO
author_rank: Newbie
timestamp: July 29, 2017, 06:33:26 am
content: Thanks, amspire. That's a lot of excellent information.I cobbled up an adapter and buffered pin 1 through an LS244 that I happened to have laying around. The software reports success with that setup. This lends support to the bad programmer pin 1 theory.The schematic you provided indicates that pin 1 is connected to Port C of the PIC and pin 40 is connected to port B. Both ports B and C are characterized as "high drive": "Suitable for direct LED drive". B and C appear to be fairly equivalent. I take "Suitable for direct LED drive" to mean that it could either sink or source fairly significant current. The spec sheet says "Maximum output current sunk by any PORTB and PORTC I/O pin ....25 mA" and "Maximum output current sourced by any PORTB and PORTC I/O pin ...25 mA"There are optional pullup resistors on these ports. Maybe if one port/pin has the pullup activated, that might make it more difficult to pull down the TTL input. Seems like a stretch since they're described as a "weak pullup".So far, the information at hand seems to point to a defect in pin 1 of the programmer.If somebody happens to come along and say that they successfully tested a 7493, I'll know for sure that they can work. But, at the moment, I'm very inclined think this particular unit is defective and needs to be exchanged.I haven't seen anything else I can lay hands on with this level of functionality in the $50 neighborhood. It's cheap...like me.

author: amspire
author_rank: Super Contributor
timestamp: July 29, 2017, 06:47:24 am
content: They could use the grounding transistors on each port to pull low instead of the PIC output - I just do not know what they use. If they use the transistors to pull low, repair would be easy.If they are using the PIC directly to drive the pins, then it is probably best to assume that it is not a great TTL tester.That IC normally has a 3.3V supply maximum and an absolute maximum of 4V. You said that the high level was 4V so that could be either the PIC output if the PIC's supply rail was at absolute maximum, or the pin's transistor high and low drivers. Since the chip can handle 5.5V inputs on the PIC even with a 3.3V supply, this would work.

author: BigO
author_rank: Newbie
timestamp: July 29, 2017, 02:33:13 pm
content: Oh, right. I wasn't thinking it all the way through. Didn't look at the rest of the schematic. The PIC does not directly drive the pins. With transistors driving the output, the PIC capability on the pins in question would be practically irrelevant.They don't specifically call it out as being a TTL programmer, so there is that. They only explicitly reference HC and LS flavors of chips supported for testing. As it's apparently an array of theoretically identical transistors controlling the inputs/outputs, it may be an aberration that the one clock signaldidwork.Makes me wonder if the thing were more open if one might be able to cheat by engaging the programming pulse circuitry to gain a little extra oomph on the pin instead of regular "output". Not sure if a 5v programming voltage would be a thing. I haven't seen any evidence of user-definable programming for this unit. From that perspective, the Wellon, though more expensive, might be worthwhile.

author: amspire
author_rank: Super Contributor
timestamp: July 29, 2017, 03:17:02 pm
content: If it is the transistors pulling low, they should be able to do TTL easily. That is what I meant by if it is the transistors pulling low, it would be easily fixable. It wouldn't be too hard debugging and fixing a transistor circuit.

author: BigO
author_rank: Newbie
timestamp: July 29, 2017, 04:32:25 pm
content: I understand and I agree with what you're saying.Per the schematic transistors are used in each of the VDD, ground or VPP circuits that can be switched on to the pin. I would assume that the transistor would assure a reasonable current drive capability, but I haven't looked deeply enough to know what to expect of these specific transistors. However, it does seem a lot more likely that there would be one weak transistor rather than one especially strong transistor in the mix.This kind of repair is within my skill set, but since this shipped locally from an Amazon distribution center within just a couple of days (as opposed to slow boat from China), I may return it as defective and try another one. If the return process is too painful/expensive, I'll look into getting a replacement transistor and give that a try.I failed to mention that I just got this unit 2 days ago.For the moment, I can test my 7493's by buffering that signal from the programmer.Thanks for sharing your valuable knowledge and insight.

author: BigO
author_rank: Newbie
timestamp: July 29, 2017, 07:19:14 pm
content: I'd still be happy to have somebody prove that the test does work on another TL866 if there's somebody out there who happens to have the right parts sitting around.