Article 5041 of vmsnet.networks.tcp-ip.cmu-tek:
Stan Gierlotka wrote:
> I am a bit suspicious about hardware-flow-control issue.
> What else can I do? What should I look at?

Do not trust the modem indicators ... stick a breakout box or monitor
box between the VAX modem and the VAX.  Clarify the settings of flow
control that the VAX modem has.

I use a 14k4 modem to a VAXstation 4000/VLC.  When I was using
OpenCMU/IP (I have since upgraded to UCX, it's free for me), I had the
following terminal settings ...

  /permanent -
  /nodec_crt -          ! prevents characters transmitted by SET
  /nomodem -		! note my VAX was the dialling end, not dialled
  /nohangup -
  /commsync -           ! enable RTS/CTS flow control from modem
  /nottsync -           ! ignore XON/XOFF flow control from modem
  /nohostsync -         ! disable XON/XOFF flow control from host
  /passall -            ! perform no translations
  /nowrap -             ! because it happened to work when I did it
  /speed=19200

Also, the modem was set for 19200 signalling rate to the VAX.  If I left
it at defaults, it would try for 9600 baud or 14400 baud.  Ouch.

Now, with the VAX talking at 19200, and the modem talking at 19200,
but the modem running at 14400, the only flow control issue is the VAX
sending too much to the modem.  So I configured the modem to assert flow
control, and configured the VAX to watch the RTS/CTS flow control
signals on the modem interface.

I could only do this with TTA2 on a VAXstation 4000/VLC, due to the
absence of these input lines on the TTA3 connector.

I also had to change a SYSGEN parameter.  I set TTY_SCANDELTA down to
1000000.  If you think about it, this parameter is calibrated in 100
nanosecond units, which is 0.1 microseconds.  One million of them is one
tenth of a second.  The default was one second.  This parameter
specifies how often the driver checked for RTS/CTS being asserted by the
modem.  If you check once per second, then the VAX could send up to 1920
bytes at 19200 baud before noticing that it should stop.  I set this to
one tenth of a second, because I found the modem had a buffer somewhere
between 1920 bytes and 192 bytes.

--
James Cameron
(cameron@stl.dec.com)
Digital Equipment Corporation (Australia) Pty. Limited A.C.N. 000 446
800