Switching to rear input through SCPI? - Page 1
Forum section: electronics / testgear

author: Svgeesus
author_rank: Regular Contributor
timestamp: May 04, 2023, 12:12:47 am
content: When measuring manually on my Agilent 34465A I often switch between the front and rear terminals. For example when measuring linearity on a voltage controlled oscillator I will have the front terminals measuring the input voltage and the rear ones measuring the resulting output frequency.I'm slowly getting into test automation and learning about SCPI. Looking at the manual I don't see a command to select which set of terminals is being measured. There is just a command to saywhichset of terminals is selected:QuoteSo do people add/make relays to switch the inputs as an external piece of equipment?Or does everyone just own multiple, high quality DMMs?

author: Someone
author_rank: Super Contributor
timestamp: May 04, 2023, 12:55:43 am
content: There is a different category of instruments for that at Keysight, 34970A -> DAQ970AWhere as Keithley has the option available for "regular" multimeters like the 2000 and DMM6500

author: alm
author_rank: Super Contributor
timestamp: May 04, 2023, 06:40:03 am
content: There are DMMs that allow you to switch between front and rear inputs remotely, but the more general solution is a scanner card with a bunch of relays, as Someone wrote.

author: coromonadalix
author_rank: Super Contributor
timestamp: May 04, 2023, 07:06:26 am
content: doing this on a mechanical switch will cause wear and tear ...... use dedicated meter with scanner cards as written aboveor use front plug with an externally controlled relays etc... i never use the front back switch, simply to avoid problems or bad contactsSince some known brands use pcb traces as switch contacts cough cough stupid ....

author: Jeroen3
author_rank: Super Contributor
timestamp: May 04, 2023, 07:28:57 am
content: Except, the DAQ970 doesn't go above 300V or 1A...If you want the ratings of the 34465A you'd have to do your own relay matrix...

author: tszaboo
author_rank: Super Contributor
timestamp: May 04, 2023, 12:00:39 pm
content: The 34465A has a clunking switch (as Dave calls it) to switch the terminal, it's not software settable.They sell scanner cards, and even switch units. Something like the U2751A.Of course you can build your own one, it's really not much effort. You can connect it to the two coax output on the back of the meter, the VM complete and the trigger, and turn it into an automatic scanning unit. You can even do this with simple logic gates, don't even need a microcontroller for it, just a 74HC4017 connected to some relay drivers.Of course test automation is a whole job that people can spend their entire career doing just that.

author: Berni
author_rank: Super Contributor
timestamp: May 04, 2023, 12:19:58 pm
content: Yep most multimeters have a physical switch on the front panel that changes the contacts over, so the software can't switch it.The real way this is done in automated test systems is multimeters that have a card slot in the back for a relay switching card. Or for larger systems there are standalone switch mainframe boxes that hold large numbers of relay cards:You can sometimes find these for cheep on ebay, since they are not the most sought after instruments.(Tho the actual relay cards tend to be the valuable parts)

author: Greybeard
author_rank: Frequent Contributor
timestamp: May 04, 2023, 12:34:58 pm
content: Quote from: coromonadalix on May 04, 2023, 07:06:26 amNever using the front/back switch can cause contact problems, too.Before I calibrate a DMM I always alternate the front/back switch 5 to 10 times to remove oxide from the unused contact surface.

author: Svgeesus
author_rank: Regular Contributor
timestamp: May 08, 2023, 04:20:46 pm
content: Thanks all for the helpful replies!I guess I need to look into a small switching unit. I don't need lots of inputs or super high speed. Honestly 2 to 4 2-wire inputs would be fine. Are there specific armature relays that people use to DIY this?

author: Berni
author_rank: Super Contributor
timestamp: May 09, 2023, 06:21:32 am
content: Nothing special really, any relay will do.For fast switching and cycle endurance often reed relays are used. They don't need to switch a lot of current so they are a good fit.If you use 2 wire resistance measurement a lot and need a very repeatable contact resistance you can go to mercury whetted relays too(only if you want to go fancy and exotic).

author: tszaboo
author_rank: Super Contributor
timestamp: May 16, 2023, 11:09:21 am
content: Quote from: Svgeesus on May 08, 2023, 04:20:46 pmFor measuring voltages, best is to use those small 2A rated gold plated signal relays. If you measure low voltages, say 100mV or so with high accuracy, then you need latching relays. The coils heat up, causing thermocouple effects.