Bench testing - Page 1
Forum section: electronics / beginners

author: johndon2000
author_rank: Contributor
timestamp: June 13, 2013, 11:32:40 am
content: What is the best way to bench test a board?Obviously, you would first check Voltages, clocks, resets, LED's working etc... but can you do much more than that?Is there a simple method of testing 74 series glue logic, Memory devices, Processors etc in circuit?

author: dfmischler
author_rank: Frequent Contributor
timestamp: June 13, 2013, 12:10:17 pm
content: It depends on what you know about the board. You can do a lot more if you have schematics and layout drawings and chip datasheets than you can without.The vintage and integration level of the parts makes a big difference. If you have a microprocessor with only 8 pins and the only thing coming off the chip is I/O then there isn't much to look at. If it's an older board with external address decode logic, timing one-shots, and other glue logic then you can check all that stuff if you have the necessary information. Old time depot repair guys had tons of tricks for quickly checking for common failures. One trick that really impressed me in the early 80's was a microprocessor chip whose data bus had been strapped to a NOP (no operation) instruction so that it would just count through all of the address range for checking address decode logic, etc. I don't know if doing that to a CPU for one board would have been cost effective, but since they were using the same microprocessor chip on everything it worked out great.As far as testing 74xx glue logic goes you can easily see if the outputs are going up and down with a scope, and see if they match the inputs (at least for the simple chips). A four channel scope lets you check gates and small decoders/multiplexers but you need more channels to really test bigger chips (think logic analyzer). In practice, the typical depot repair guy in the 80's had access to a 2 channel scope and it was cheaper to replace a chip that acted funny than it was to spend tons of time trying to figure out what was wrong with it. Most of the places I worked had somebody whose job it was to write manufacturing test software that tested individual circuits and that software was also often useful for depot repair diagnosis.

author: Anks
author_rank: Frequent Contributor
timestamp: June 13, 2013, 12:14:49 pm
content: Get a copy of the 74 logic handbook and use your scope or logic probe to check for correcty behaviour and that nothing is tying line high or low.For processors check that the address and data lines are being driven and for bus contention.Same goes for proms and memory.

author: Anks
author_rank: Frequent Contributor
timestamp: June 13, 2013, 12:27:48 pm
content: You can get the TTL handbook pdf from my blog I also have a onsemi version somewerehttp://krisanks.wordpress.com/2013/06/13/texas-instruments-ttl-handbook/

author: johndon2000
author_rank: Contributor
timestamp: June 13, 2013, 02:06:09 pm
content: Quote from: dfmischler on June 13, 2013, 12:10:17 pmThanks for that, very helpfulUnfortunatly I don't have access to schematics, layouts, or a logic analyser! All I have is access to datasheets for individual devices...The processors I am looking at are MPC603, MPC750, M68020 etc.Not too sure how I'd go about looking at the address/data lines as the board isn't connected to the rest of the system so I guess I can only check voltage levels/ resets etc?Thanks again

author: johndon2000
author_rank: Contributor
timestamp: June 13, 2013, 02:06:37 pm
content: So, for the logic gates is the best idea to use a logic pulser(s) on the input & a logic probe on the output?! Vcc and GND are supplied, but other than that there are no inputs as it's not connected to a system (so no address, data, control etc).Thank you

author: dfmischler
author_rank: Frequent Contributor
timestamp: June 13, 2013, 02:43:29 pm
content: It sounds like you are trying to troubleshoot a system that includes several boards and a backplane or card cage. If so, it would probably be best to work on them as part of the system. Is this a well-known bus, or at least a common connector system? If so you can probably find a bus extender card (e.g.thisorthisfor PCI). These extenders get the board up off the connectors so you can probe it, and may also break out the bus signals so you can easily monitor them while the system is operating.I would probably not waste my time checking every individual gate on a board unless there was a big economic incentive and I had no way to make the board operate so I could watch it screw up. A few times I have actually made my own schematic from a board so that I could troubleshoot it, but this is a lot of work and might be infeasible if there are more than two layers.Perhaps you would get better answers if you provided more complete background information on what you are trying to do.

author: Anks
author_rank: Frequent Contributor
timestamp: June 13, 2013, 05:01:24 pm
content: Have you got a picture of what sort of thing you are on about trouble shooting. If its old logic boards like a C64 then checking logic and memory is the way to go. Well after voltages and clocks.If its a high speed bus then you will need better logic analysis equipment.

author: johndon2000
author_rank: Contributor
timestamp: June 14, 2013, 07:24:07 am
content: Thanks guys.The board is a NT9X35BA (not too sure how to upload pics but if you google it you'll see what it is - its a ENET crosspoint card, out of a telecomms system... the system just flags it up as faulty, no other diagnostics... I don't get bogged down in all the telecomms stuff (i.e. how it works in the system), just the electronic aspect (it's faulty, needs repair!).There is not a common bus, and I'd need to get an extender card made up which would cost a few hundred pound/ dollars.Thnak you