[Simh] Problem using a real VT terminal with simh

[Mark Pizzolato]

On Tuesday, December 5, 2017 at 10:08 AM, Paul Koning wrote:
> Do you have flow control (Xon/Xoff) enabled both on the terminal and on the
> VAX?
> 
> You might try a slower speed to see if that makes a difference.  It shouldn't,
> but it might give more information.
> 
> Is autobaud enabled on the line?  You might turn that off and set the speed
> and related parameters explicitly.

These are all great suggestions.  Autobaud should definitely be turned off on 
the VMS side, but I don't think that is the problem since he's able to login
and the commands he types produce output, which merely is garbled.

A slower speed, which is explicitly stated on the ATTACH command might
help.
     sim> ATTACH DZ LINE=0,CONNECT=SER0;2400-8N1

I think that what you're seeing is output from the simulator overrunning 
the host OS provided interface to the serial port.  This came up a while back.
The simulator serial port interface delivers data to the host OS driver one
character at a time as it is emitted by the simulator.  The simulator is 
notified of I/O completion when the host OS said the IO was complete.
On some operating systems, this completion notification merely indicated
that the data had made it to a kernel buffer and not actually out through
the hardware.  This early completion indication then was passed back 
through the simulator and more data was then emitted.  Ultimately it seemed
that there was not a reliable way to know that the data has actually, not only
made it to the hardware, but actually been transmitted.  Some relatively 
common serial ports have an output buffer in the hardware that will take 
a number of characters to be transmitted as quickly as the host puts them 
into the hardware, and then deliver it outbound after the fact.  In any case
due to the problem of not getting a precise indication from the OS driver of
when the data actually made it out the serial port, an alternative approach
was taken.  This approach attempts to time the delivery of character I/O 
data to the OS driver by explicitly scheduling the I/O completion time based
on the port specified speed.  This approach presumes that the OS actually
starts to send some of the data to the serial port when it is presented and 
doesn't hold off until some buffer full condition is achieved.

If the above ATTACH suggestion doesn't work for you, please create an issue
at https://github.com/simh/simh/issues and we can try to dig into this further.

> > On Dec 5, 2017, at 12:56 PM, Peter Allan <petermallan at gmail.com> wrote:
> >
> > I am trying to get a real VT420 terminal working with a simulated micro VAX
> 3900. However, I am having problems getting text displayed on the screen of
> the VT420.
> > ...
> > Once the microVAX has booted, if I hit Enter on the VT420, then I can see
> that a login process starts on the simulated microVAX, but all I see on the
> screen of the VT420 is a few garbage characters. Despite that, when I type a
> username and a password, I do get logged into the simulated microVAX. Any
> commands that I type on the keyboard of the VT420 appears on the screen of
> the VT420 (except the password, of course), but anything that the microVAX is
> sending to the VT420 appears as garbage. There are recognisable characters,
> but if I TYPE a file, even a small one, what appears seems to be every 7th
> character from the file starting with the first character. The output of
> commands such as DIR appear to have the same problem. ...
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