I have been struggling with getting my modems configured properly for
my DTC modem ports. I have the modem DTE rate locked at 19200 baud,
which is the fastest speed at which the DTC will operate. Hardware
flow control doesn't work on this particular old model of DTC.
Software flow control works, but only sort of. Here is how I think it
works.
 
We all know that when the HP3000 is sending stuff too fast for us to
read, we can do ^S to stop the data, and ^Q to resume it. This shows
that the HP3000 does software flow control, using XON (^Q) and XOFF
(^S). However, this only proves that the HP3000 provides this flow
control in the transmit direction (from the HP3000 to your terminal),
listening for and taking action on your XON/XOFF commands. The weird
part is that it appears to me that the HP3000 does not use any flow
control at all in the receive direction (from your terminal to the
HP3000).
 
I configured my modem to act on XON/XOFF characters sent by the
HP3000 to the modem, on the theory that these fast modems might send
data to the HP3000 faster than it can digest, and that the HP3000
would signal the modem to slow down by sending an XOFF. This caused
no end of trouble, particularly with the terminal status checks that
Qedit does. The usual result was that the terminal keyboard stayed
locked, and Qedit would assume that it was working with a non-HP dumb
terminal. I finally setup the modem to send XON/XOFF characters to
the HP3000 as required, but not to listen for them from the HP3000.
This seems to allow most everything to work as desired.
 
I am left with the nagging worry that as these faster modems (> 19200
baud) become more common, we will eventually run into cases where the
HP3000 cannot digest the data quickly enough, but cannot signal the
modem to pause and resume.
 
Is my concern justified, considering that the baud rate between the
modem and the HP3000 is only at 19200, even if the two modems
communicate at a faster rate?
 
Regards, Mike Shumko, Robelle Consulting, 1-800-561-8311