ATV61 Speed Reference Problem

Replaced Altivar 58 with ATV61 40HP drive. Everything works except speed reference from AB SLC505 PLC. Output is isolated using Spectrum Controls AO card model 1746sc-INO4i.

Now here is the interesting part. When connecting output to drive with proper polarity, output goes to 50mA even though PLC has been adjusted to source 12mA and VFD runs at 60Hz. When connecting to drive with reverse polarity, output is stable at 12mA, but drive of course runs min speed of 40Hz. Input resistance to drive is 250 Ohm. When running speed reference from loop calibrator, VFD runs fine. Checking PLC output with calibrator, 4-20mA measures OK as well. Card runs multiple VFDs. Some Altivar 58 and AVT61s. Altivar 58s run fine and don't exhibit this problem. Frequency also remains stable when ATV61 is connected.

I've talked to Spectrum Controls and Schneider Tech support and everybody is scratching their head. I'm at my wits end as well. Output card is already isolated. A loop isolator I don't think with help here. What is the problem? Who can this be checked further?

Thanks everyone,