New (to me) HP 3421A DA/C Unit, some questions

[4ster]

Printer problem was solved: I assumed that the FP program on the module automatically set flag 21. It doesn't. I must have cleared flag 21 at some point with another program. (I thought I tested this flag setting before my initial post.)

It makes sense that the user can control the printer by not having a default SF 21 in the FP program.

Can someone familiar with the module explain the function of the CMD key on both the Front Panel and Data Logger overlays? The manual is mute about its intended use as far as I can determine. I had thought that it might be an unassigned key that could be assigned to a custom program - its not. If it is meant to run one of the sub-routines on the module, I have not been able to figure out how it works.

Steve