28 Feb
2013
28 Feb
'13
midnight
On Wed, Feb 27, 2013 at 12:53:38AM -0800, Earl Evans wrote:
Regarding the comments on baud rate and flow control -
still doesn't make
sense to me why the VT100 would not exhibit the error with a real PDP-11
(at 9600 baud), but does do it with the emulated PDP-11. The info is coming
at the terminal at the same rate - why would the behavior be different?
My guess is buffering. The serial driver and/or host OS are likely to
provide some buffering on output so that the first N characters sent over
the emulated DL11 appear (to the PDP-11) to go out more or less instantly.
So by the time an XOFF comes back (and the PDP-11 stops sending more chars
to the DL11), lots more characters are already working their way out, so
you're going to get the SUB blocks on the screen when they arrive at the
VT100 and overflow its input buffer.
I don't know what other emulators have, but in E11 there's an ASSIGN /TXMAX:n
switch which sets the limit for transmit buffering. I think the default is
something like 16, which is OK for some devices which send XOFF when they
still have space left, but not for others like the LA120 which needs /TXMAX:1
or else BOOOOOOOOOOOOOOOP.
John Wilson
D Bit