where to attach probes? - Page 1
Forum section: electronics / beginners

author: animaal
author_rank: Newbie
timestamp: May 04, 2019, 08:56:40 am
content: Apologies in advance for what is probably a stupid question. I'm new to the world of electronics and I haven't yet seen it covered in any other posts or Youtube videos.I'm thinking about how useful an oscilloscope would be, to play with (and maybe create) simple peripheral devices for old computers.So I can see how a 4-channel oscilloscope would be great to see what's happening, using one channel on the Write line, one channel on the IOREQ line, perhaps a channel on one of the address lines, and who knows, maybe a channel on a Read or Chip Enable line.On an old 80s era Z80 board, I think I could use crocodile clips to attach the oscilloscope channels to the legs of the big components.But I wonder, how is this done with newer equipment? Equivalent devices created after the 80s (e.g. tablets, laptops etc) tend to be more cramped, and use a lot of small surface-mount elements. I can imagine carefully(!) holding a probe to a point on a board, maybe two probes at a push. But I have a limited number of hands. How do people manage this in real life?Thanks.

author: Ian.M
author_rank: Super Contributor
timestamp: May 04, 2019, 11:40:00 am
content: One finds a robust enough pad on the trace of the signal you want then solder on a little loop of wire as a testpoint you can clip the probe tip onto.However if you are working with old computers from the pre PC or even PC-AT era, with mostly parallel busses, the correct tool for the job (once you've verified that individual signal levels are valid with a scope), is a logic analyser, and you typically need to put a test clip on the CPU, or a memory chip, or put a breakout card in a bus slot to get pins you can connect the many logic analyser probe wires to.Modern computers have fewer parallel busses, and those they do have are typically high speed and impedance controlled so you typically cannot simply probe them without upsetting operation. Depending on the bus speed you may be able to solder on individual very short wires (typically kynar wirewrap wire) going to a connector to fit your logic analyser pod.

author: ebastler
author_rank: Super Contributor
timestamp: May 04, 2019, 11:48:04 am
content: There are also various arms and clamps for holding and positioning probes relative to your PCB. A Google image search for "oscilloscope probe positioners" gives an idea of what's available.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 04, 2019, 12:15:25 pm
content: depending on which oscilloscope we are talking about...sorry those are not my pictures so cant resize...

author: animaal
author_rank: Newbie
timestamp: May 04, 2019, 12:52:45 pm
content: Thanks, that's all very interesting.It looks from the last image above that all electrical engineers should invest in a packet of good quality clothes pegs

author: DDunfield
author_rank: Regular Contributor
timestamp: May 04, 2019, 12:59:17 pm
content: Quote from: animaal on May 04, 2019, 08:56:40 amExactly why I built the "Self Holding Probe" shown below.Don't laugh.. Ok, It may not have the best high frequency response or noise immunity, but it's been a surprisingly useful bit of kit.The "business end" is a sewing needle - very fine and very sharp. Quite easy to position on even the smallest of traces.Dave

author: bdunham7
author_rank: Super Contributor
timestamp: May 04, 2019, 01:23:32 pm
content: The simple answer is that it isn't easy or simple. Test leads and jigs can be a significant portion of your test equipment outlay and each situation calls for its own solution. And as Ian mentioned, it can also be problematic to probe high-speed low-power circuits due to circuit loading--even small parasitic capacitance can alter a data bus or a crystal oscillator circuit. If you are doing R&D, the best solution is usually to buy or make a custom connector for each application. If you are a repair tech this may be impractical and you'll need to be creative!