Ch 3 on 6th mod of 1121s w/ 1321 FE gives bad readings if not scanned by itself

[Tim Jones - Space Shuttle]

I can scan channel 3 by itself or with channels from another 1121 module and recieve a good reading but trying to scan it with the other channels from the same 1121 module results in a bad reading. I do not have this problem with any of the other 1121 modules. There is an 1141 with a 1305 in slot 7 of the chassis.

[Spencer S.]

Tim,

Can you scan the remaining channels from the 6th module together without issue? When you receive the bad readings from Channel 3, do the other channels from the module return correct data? What is bad data? One thought, I have seen some strange behavior when the gain jumpers in an SCXI-1121 are not set to the same values that are specified in MAX. I would verify that all of the gain jumpers correspond to the values in MAX.

Spencer S.

[Tim Jones - Space Shuttle]

Spencer, the problem turned out that when using channel strings to read the shunt value, you get bad readings. The root cause is that when you read ONE shunt channel, all it good, but when you read the combined shunt channels from 1, 2 and 3 on the same 1121 module, in a SINGLE AI-1Scan, (an array of shunt strings as channel names), the channel 3 value is always bad.

Basically, if you can get away with reading the shunt channels with separate scans, you have accurate results.

(NI, shouldn't we be able to send an array of shunt strings to the AI-1Scan without error?)

[Tim Jones - Space Shuttle]

I know it has been a long time since the last post on this issue, but I wanted to give an update.

It turns out that all 14 of the 1321 front ends I purchased had solid state relays from the same lot (4 per module.) All of these relays with lot 0027T1267 relays from Clare do NOT meet manufacturer's specs for temperature, and so can cause erratic problems. At a mere 96F, they change values and eventually completely change states. Since they aren't mechanical in nature, they don't hard switch on or off, but rather have inbetween values that can appear to be bad readings. See my other post on 1321's for more.

Tim Jones