Garbage on screen when using real VT100 on emulated PDP-11 - test-drb@ccmp.vtda.org

27 Feb 2013

...

 OK, I'm stumped.  I'd like to use a real VT100 as a terminal with an
 emulated PDP-11 running RT11.

 When the VT100 is connected to a real PDP-11, it works fine. When connected
 to the emulated PDP-11, garbage characters begin to appear on the screen
 when, say, doing a DIR command.

 The garbage always starts in exactly the same place in the DIR listing.
 After that, garbage characters (grey squares) become interspersed with the
 good text. 
 Too high a data rate without flow control.  Either enable Xon/Xoff on 
 both ends or
 use hardware control (most PCs do not support that correctly).

 If all else fails set the baud rated to something slower till the 
 problem goes away.

My VT102 manual describes the grey hatched squares as the substitute character
and says they are displayed when the input buffer is full and characters are
lost.

It also suggests that hardware flow is not available and the only solutions
are XON/XOFF, fill characters (nulls) or low speed operation.

I strongly suspect the VT100 behaves the same. Appendix E "VT102/VT100
DIFFERENCES" doesn't suggest otherwise although it does say that the VT102
uses a different numbers of fill characters.

Regards,
Peter Coghlan