3.8 I'm trying to upload a file, via WRQ, over a modem dialup. I'm dialed into ldev 21, so this means I'm not coming in over a DTC, but the internal mux port. I get the message "Host not responding".

I've tried enabling/disabling host prompt, enq/ack, and DC1. I've tried using the pre-configured "transfer to HP 3000", "transfer to HP 3000 over modem", "troubleshooting transfer to HP 3000", and "troubleshooting transfer to HP 3000 over modem". I.e., I believe I've tried the obvious, and a fair amount of the non-obvious.

You have to specify:

transfer to HP 3000 over X.25

Of course you are not actually using X.25 and probably do not even have the product, so this is annoying not to mention rather non-obvious. LDEV 21 is handled by a different set of drivers than DTC ports or VT ports. And, it has somewhat less capability as demonstrated here.