Testing TTL in-circuit - Page 1
Forum section: electronics / testgear

author: caius
author_rank: Regular Contributor
timestamp: June 15, 2013, 05:30:45 pm
content: Hi all,just wondering if exists an effective way to testing TTL in-circuit (without desoldering them).I'm not talking about logic comparators, I have three of them but, IMHO,they are not very reliable.I heard about Huntron or ABI elecronics troubleshooters but they are out of my budget.Could some give me some advice?Thanks.

author: edavid
author_rank: Super Contributor
timestamp: June 16, 2013, 12:44:28 am
content: This is what a logic pulser such as the HP 546A is used for - to drive a TTL node to the opposite logic state for testing.It is also the purpose of the output high short circuit current spec in the datasheet - that tells you how much current your logic pulser needs to sink.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 16, 2013, 03:24:40 am
content: In my experience,TTL either works,or it doesn't.Can you leave it in place in the equipment & trouble shoot step by step with an Oscilloscope?If you can,& have another unit you can compare with,you should be able to track the problem down.If you need to test a board out of the equipment,you are then stuck with either supplying all the operating voltages & pulses necessary,or just doing resistance tests on the dead board.The advantages of logic ICs compared to analog circuitry is that there are very few external comonents "to muddy the waters".For instance, if you find an IC with a totally different reading in gate resistance to others of the same kind,the device is probably faulty.All the digital purists will probably faint at the above,but I have successfully fault-found plenty of TTL circuitry using these methods

author: fpga
author_rank: Regular Contributor
timestamp: June 16, 2013, 03:31:10 am
content: Ah, the good old days... this is where the logic probe and logic pulser came in handy. You looked up the part pinout and function in a datasheet, probed the levels of the inputs and the output of a gate to determine if the output was correct based on the inputs, then used the pulser on the inputs one by one while observing the output. If you observed a pulse on the output, then chances it was good, and no pulse meant probably bad. I once used this technique to fix an 8086 PC motherboard, ended up only changing one part, and fixed the motherboard. Back then PCs cost several thousand and I was a student.

author: SeanB
author_rank: Super Contributor
timestamp: June 16, 2013, 09:38:28 am
content: Trust me that TTL will fail in weird ways. They can fail such that they pass a DC and low frequency logic test, but will not work with a input within the spec for frequency. They can also have logic threshold level changes that make them not work correctly in circuit when supplied by levels that are not going to operate the chip but which are within the level specs. They can also have delay changes that take them out of the datasheet specs. Had a lot of 7474 flip flops that did this with time, you just changed them if they were in a block of non working logic on sight. Not a manufacturing problem but a design issue.

author: caius
author_rank: Regular Contributor
timestamp: June 16, 2013, 03:25:34 pm
content: Quote from: SeanB on June 16, 2013, 09:38:28 amYes, I know, threshold level changes are the most difficult faults to locate!It happen when the chip is still functioning but its thresholds are wrong, it changes as it should but is no longer latching cleanly or on time.I know all the traditional methods to test the TTL (diode test, logic probing, etc.. but, for my job, I have no time to manually test every chip and this is problem when I did't know how work the system under test and I don't hanv any schematics of it or have the time to figure it out.So, I was looking for some tool who could do the job more or less pointing me in the right direction al least.

author: SeanB
author_rank: Super Contributor
timestamp: June 16, 2013, 03:42:13 pm
content: There are TTL comparators, that use a test clip and you plug a matching known good IC into them, then they run the 2 chips and tell you pass/fail by detecting logic level variations and pulse delay for each pin. Not perfect ( they do not do well with RC delays on inputs or with monostables) but they do at least do a functional test of how the chip operates in circuit. I have used one, and you need to use the right family as the known good chip, if you have standard TTL you must not use a LS or S or even C in the socket, as it will falsely say the DUT is faulty in a lot of cases with a fast clock.

author: caius
author_rank: Regular Contributor
timestamp: June 16, 2013, 09:14:23 pm
content: Quote from: SeanB on June 16, 2013, 03:42:13 pmYes, as I said, I used logic compatator since many years, I have an HP 10529A and two B&K Precision (models 552 for CMOS and 550 for TTL),But as you said, they are not 100% reliable.For examplre just today I repaired an arcade board (yes, I work also on this vintage stuff) which had the 68000 CPU in /HALT state.First with fluke 9010A troubleshooter, I tested all the work RAMs and they were good.So, the missing program code execution which caused the /HALT state had to be due a bad buffer between CPU and program ROMS.OS, I tested the buffers with the B&K 550 logic comparator (it can test also up to 20 pin devices) and they were all good!It was impossible!Infact I desolder one by one the buffers and tested in my EPROM programmer: found a bad 74ls245, changed it and PCB fixed!So, are you sayng that when I test 7LS TTL series with logic comparator , I must not to use same 74LS series as comparison IC?What do you mean for "standard TTL "?

author: SeanB
author_rank: Super Contributor
timestamp: June 17, 2013, 05:00:50 am
content: If you are testing eg 74LS74 use a 74LS74 in comparison socket, not a 7474, a 74S74 or 74C74 or 74ALS74 or even a 74HCT74. There are slight differences between them timing wise and threshold wise, which can trigger the tester to say the device is bad. All the devices are the same logically and pin out wise, but have different switching thresholds ( but are all TTL compatiable) and slightly different delays from inputs to outputs, and some have slightly different actions on certain logic combinations that are normally marked do not use.

author: notsob
author_rank: Frequent Contributor
timestamp: June 17, 2013, 05:21:29 am
content: If you have a good board, (and the patience), you can do a side-by-side comparison of the gates by looking at the diode signatures of the pins with an 'octopus', BUT its not going to show you timing problems.

author: caius
author_rank: Regular Contributor
timestamp: June 17, 2013, 06:19:00 pm
content: Sometimes I use also the diode test method with a multimeter : red probe on GROUND and black one on each pin, you have to measure on them from 0.5 to 0.9 mV, less or more than these values and there are chances the gate is bad .I admit, it's a very rough method but sometimes it works (only to discover short (or quite short) and open circuit, obviously).

author: fpga
author_rank: Regular Contributor
timestamp: June 17, 2013, 07:12:36 pm
content: If it is a failure that shows up when the device warms up, try the freeze spray. Test the unit or board when it is at room temperature. Does the problem exhibit itself? If it works cold but fails as it warms up, then keep on testing while hitting each TTL part with freeze spray one by one while testing. Ideally you'll find that on one part it starts to work again and then fails again as the part warms up. Parts which fail at frequency or have bad trigger levels or bad drive characteristics tend to be very sensitive to temperature.If temperature has no effect, then I think the logic probe and logic pulser is still a good option, though it will take a lot of time particularly if you don't have schematics to follow.If the parts are socketed then you could go the swap route provided that you have one of each of the parts that are used.Try to analyze the nature of the failure as this could lead to localizing the area of the board that is most likely to have the failed part. Look up the part datasheets to identify their function and pinout. Sometimes you can improve your luck by trying the parts that seem most likely to implement the function causing the failure.Signature analyzers, if you can still find a fully functional one, relied on comparing a good board with a bad board. So, unless you have a good board to test against, it won't be of much help.As for parts testers, it's not much better than doing the swap method, if the parts are socketed.

author: mailmad
author_rank: Newbie
timestamp: December 16, 2018, 11:07:31 pm
content: Logic pulsers (transmitters) and logic probes (receivers) are very good tools, however provide only one input and one output. This allow testing only for a limited number of gates, specifically buffers and inverters.ABI 8 is a very good device, however is very expensive.There is a device which fill the gap between the above two alternatives:https://www.ebay.com/itm/NEAT-Mi-in-circuit-IC-tester-VI-signature-tester-more/283301277781?hash=item41f6127055:g:pGoAAOSwUxlb6gmM:rk:1:pf:0Is at least 3 times cheaper than the ABI, provide 7 times more I/Os than logic pulsers/probes (6 transmiters, and 8 receivers), and have a special switch for TTL option.Does not work with libraries like the ABI. Mi have a built in logic analyzer. User find the condition of a gate by examining the timing diagram on screen. Seems complicated, however isn't. Just need an initial familiarization time.The downside of Mi, comparing to ABI is does not test complicated ICs like microprocessors.Mi except the in-circuit logic IC testing function, have many other. Those I personally found very useful are the following:- In circuit analog IC tester- VI component tester with variable frequency- Dual channel oscilloscope- I2C, SPI, Serial, and RS485/422 protocol reader

author: blacksheeplogic
author_rank: Frequent Contributor
timestamp: December 17, 2018, 12:20:09 am
content: Quote from: fpga on June 16, 2013, 03:31:10 amGood Old Days?Thanks very much, I still have my probe & pulser, and you know they are still available.

author: David Hess
author_rank: Super Contributor
timestamp: December 17, 2018, 04:17:14 am
content: The trick I usually start off with is using an oscilloscope set to long persistence to check voltage levels. If I see something which is crawling around between low and high, then there is something wrong at that circuit node.

author: SeanB
author_rank: Super Contributor
timestamp: December 17, 2018, 09:50:16 am
content: Another trick is to check the outputs are not shorted to supply or ground, then simply put another identical TTL IC over the suspect one and see if it works, or if it does not give the same fault indication. Sorts out those with threshold shifts and those with failed inputs that are either slow or open. TTL is quite forgiving of shorting outputs to ground short term, not so shorting to Vcc.I did make a quick and dirty clock driver by taking a few 5404 inverters and stacking 4 CERDIP packages on each other, and placing a 10uF 25V wet tantalum capacitor on the top, soldering all the pins in the stack together. Took some wires and joined all outputs of 5 stages together, and took the 5 inputs and joined them to the output of the 6th inverter. Then used it to deliver the ISA bus clock to a card edge connector, just in case it was needed, the rest was done with some 8255 POI chips on the card. Made a pretty good 14MHz transmitter, and ran quite warm, though that was pretty much usual for plain TTL anyway.