pretty stange board

We make a lot of weird products, in VME and little Ethernet boxes. They use D25, D37, SCSI50, SCSI68, and D9 i/o connectors. Some products follow sorta repeatable channel/pin assignments, and some don't.

We now have a motley collection of test sets and fixtures, some historical creaky DOS/ISA/Bit3 stuff. The new regime will be one universal test box, and Ethernet connection to the test gear, code in Python.

We'll use a Fluke benchtop DVM (the Keithleys suck) and an Agilent SMU (the Keithleys suck) and a rackmount scope, probably a Rigol. Maybe an ARB, again a Rigol.

My assignment is to design the main board, which is basically a relay matrix. It goes something like this:

formatting link

There are 68 "PinN" signals, all used on a SCSI-68 connector and less used on others. Each pin has three DPDT relays so it can be left open or connected to one of five internal busses, A B C D or E. Some of the pins can also be locally grounded; there are reasons to do that, trust me.

formatting link

Then there is a "test matrix" that can ground the busses, short them, and connect them to the test equipment. More relays. There are also eight resistors that can be connected to A B C D in 4-wire mode, or fake 4-wire mode for the higher values. They simulate RTDs and such.

formatting link

An ARM cpu manages it all, actually pretty simple since it mostly clicks relays. It's looking like I'll have about 260 relays on the board.

We'll use the TI TPIC6595 SPI relay drivers.... 32 or so of them. I figure that if I allow the Fluke to monitor the +5 supply current, we can check the coil currents and verify that the shift register and coils are OK. Actual verification of the entire test set will require some loopback connectors, which can be little paddle boards.

A test stand will need two of these boards, since many of our products have two i/o connectors.

We can build a dozen of these boards, and keep spares in the closet. Three reels of relays ought to do it.

I wish I could buy a universal crossbar switch!

Reply to
John Larkin
Loading thread data ...

A long time ago back in analog history there used to be arrays of crossbar switches, something of the order of 100 * 100 lines. Havent seen anything like that in years. I presume phone line and similar switching is now all done with CMOS analog switches, ruggedised versions of the venerable 4016 etc.

Good luck.

Reply to
Adrian Jansen

Phones are all digital. Each POTS phone line gets an ADC, a DAC, and some sort of DC feed/ringer. No more Strowger relays or crossbars.

I guess the exchanges are silent now. They used to sound like a machine-gun nest.

My exchange doesn't seem to accept pulse dialing any more.

Reply to
John Larkin

I remember the crossbar race, chip makers trying to get more and more switches on a single chip. That was about the same time telephony went digital which means it was all handled very differently with digital muxes rather than analog switches. The crossbar race ended very quickly, almost as soon as it started.

Digital is so fast, switching can be done in time much, much more efficiently than analog could ever hope to achieve.

That said, telephony is pretty impressive stuff. I have been told they have such stringent requirements for lost bits and dropped calls that even with digital stuff it can be hard to meet the requirements. If you drop 1 in 10,000 calls, how do you troubleshoot that?

--

Rick
Reply to
rickman

What's your relay MTBF going to be? Do you use a routine of switch, self check, test UUT, self test? Or all SS relays?

NT

Reply to
tabbypurr

We built several of these for remote selection of signals from areas that are inaccessibe due to radiation. It uses two 16 x 8 crossbar arrays and 8 channels of buffer amplifiers. A Beagle Board computer controls it as an Ethernet appliance.

So, you have 32 50-Ohm inputs and 8 outputs. Each output can select from any of the 32 inputs. Very handy!

Jon

Reply to
Jon Elson

#5 ESS used a huge array of ferreed switches, basically a giant core memory with biased reed switches in each core. The array magnetizes the core to either aid or cancel the bias magnet.

it made a fascinating tinkling sound when you stood in the ferreed array, thousands of contacts a second making tiny plinks as the opened and closed.

Not sure how many #5 ESS systems are still running, but I sort of think there are quite a few still running. Perhaps they have superseded the ferreeds with somthing totally electronic.

Jon

Reply to
Jon Elson

Well, a little wiki searching indicates that what I saw must have been a #1 ESS, not #5, with the ferreeds.

Jon

Reply to
Jon Elson

I'll use little surface-mount Fujitsu DPDT electromechanical relays. They have been very reliable.

Of course, we'll have to test the test set regularly, once a week maybe. It's probably that a relay failure would fail a DUT test, so we'd catch most of those.

It's really hard to beat a relay for on resistance, isolation, and electrical ruggedness.

Reply to
John Larkin

Den tirsdag den 29. september 2015 kl. 05.10.08 UTC+2 skrev John Larkin:

Agilent 34972a with the right modules could probably do it

34904A, 4 x 8 Matrix Switch 34908A, 40 channel single ended multiplexer

it measures voltage,current,resistance,frequency,period,rtc,thermistor, thermocouple and is easily controlled with SCPI on via serial port or tcpip

-Lasse

Reply to
Lasse Langwadt Christensen

Hey, they use relays!

Reply to
John Larkin

The early #5ESS used a "concentrator" in front of the line circuits to minimize the number of A to D and D to A converters needed. This concentrator was made of ferreed switches (circa 1982). With reducing costs of electronics, modern #5ESS machines don't use the concentrator anymore, although there are still some older ones in service.

The #1ESS (circa 1965) used ferreed switches as the switching fabric and was an analog switch, albeit computer controlled.

The #4ESS toll switch was the first fully electronic switch in the old Bell System, circa 1974. It had the interesting characteristic of being silent.

Eric

Reply to
Eric Tappert

GTD5 EAX?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
 Click to see the full signature
Reply to
Phil Hobbs

Surplus source for the famous Cunningham crossbar? CMOS equivalent?

Reply to
Robert Baer

You might consider having your POST exercise all the relays a number of times to help keep the contacts clean. I have a Tek CG-5011 plug in that does just that. The natural wiping action helps to clear any film that might accumulate over time on the unused relays.

On some equipment, connecting a function generator to a dirty relay can usually fix intermittent problems.

Reply to
Tom Miller

Last time I needed this sort of thing for a production test / calibration rig, I ended up buying rather than designing one.

I know Keithley are not exactly on your favoured supplier list, but is this any good?

formatting link

The one I bought has been in use for 10 years now with no reported problems. Old Keithley gear, from the days before they started re- badging cheap Chinese junk, has been pretty good IME. The same rig used a model 2000 DMM, again, no problems.

Reply to
RBlack

On Tue, 29 Sep 2015 14:43:12 -0700, John Larkin Gave us:

snip

OK, John. You always state that I am not in electronics and boast about all of your stuff. Give me an answer to this one... Should be real simple for your "great mind".

What is the best IQ Demodulator chip?

Reply to
DecadentLinuxUserNumeroUno

I never said that.

and boast

I talk about what I do (which few people here actually do), the successes and the blunders.

Give me an answer to this one...

About the only electronics that I've never done much of is RF; it never interested me for some reason. And it would depend on your frequency range and such.

That said, several people are making IQ chips that work over a really wide frequency range, like 50:1, and have internal 90 degree phase shifters. LTC and TI at least. How can they do that?

Reply to
John Larkin

A couple of our existing test rigs use K 2000 DVMs with the internal scanner option, and they seem to work fine. That aren't too good at microvolt levels (thermocouple instruments) so I plan to build a good

100x instrumentation amp into my test board.

The problem with using some commercial scanner is that I'd have a zillion wires running from our DUT connectors to the scanner. It's easier to put all the connections and switching on one PC board that I can build a dozen of.

Keithley used to be wonderful. I returned four of their 2100 (Chinese rebrand) DVMs and one of their expensive SMU boxes that had astonishing EMI problems. The replacements are Fluke and Agilent.

(It's hard to bring myself to say Keysight.)

It looks like I'll have about 260 relays. The "universal" module is a

6:1 mux that uses three DPDT relays. All sorts of things (DUT pins, instruments) can use that block to connect to the five global busses or go open circuit. We can draw that little circuit once and then cut/paste about 80 of them all over the place. It's looking like a 25 sheet schematic.
Reply to
John Larkin

I'm sure you've calculated it, so you know how often you'd need to test.

Absolutely. But large numbers of relays can bring MTBF down to impractically few hours. And where data is fragile, a slight contact crackle effectively becomes a failure. A suitable self test sequence can greatly improve that.

NT

Reply to
tabbypurr

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.