1771-RTFM
Okay, let's start from the beginning with a little bit about your architecture.
I presume you have a PLC-5, as well as a 1771-OFE2 analog output card. There are three kinds of general-purpose analog output cards for 1771 I/O, the OFE1 (0-10V), OFE2 (4-20mA) and OFE3 (0-50mA).
Any analog card in the PLC-5 communicates with the controller via special commands in ladder logic called Block Transfers. (BTR and BTW for Read and Write).
I'll ask several questions of you:
1. What model of PLC-5 are you using ?
2. Is the analog module in question in the chassis with the PLC-5, or in a remote I/O chassis on RIO or ControlNet ?
3. Are there other analog modules that work, and are they near this module in the chassis ?
4. Is this a new program and installation, or was this working in the past and stopped working ?
You'll find that your block transfers are controlled by "BT" data structures (in modern PLC-5's with Remote I/O. PLC-5C ControlNet and old PLC-5/25 and 5/15 are different). When you set up a BTR or BTW, you enter the Length of the block transfer in Words, and the instruction echoes back the length if the BT was successful. If the BT fails, the BTR or BTW instruction returns a negative number in the Delivered Length field (.DLEN in BT parlance), the value of which designates the error code.
The method I use to trap a periodic block transfer failure is to count both .DN and .ER bits in the block transfer control file.