[Simh] Problem using a real VT terminal with simh

[Mark Pizzolato]

Hi Peter,

Even if you do solve the problem by some different combination of things, the problem you’re seeing is real and possibly generic, so understanding the differences between when it failed and when it worked may lead to a different approach which works more universally.  So, please create an issue either way.

Meanwhile, you might try a different simulated device for this activity.

Instead of:
      sim> ATTACH DZ Line=0,CONNECT=SER0;2400-8N1
please try:
      sim> SET CONSOLE SERIAL=SER0;2400-8N1


-          Mark

From: Peter Allan [mailto:petermallan at gmail.com]
Sent: Tuesday, December 5, 2017 12:39 PM
To: Mark Pizzolato <Mark at infocomm.com>; simh at trailing-edge.com
Subject: Re: [Simh] Problem using a real VT terminal with simh

Thanks to everyone for their quick responses.

To my surprise, Xon/Xoff was turned off on the VAX, specifically, SHOW TERM TTA0 showed No HostSync. However SET TERM/PERM/HOSTSYNC TTA0 made no difference.

I have tried setting the line speed to 2400, but that only slows down the output. It still contains the same characters, to the extent that I can tell.

I will continue to experiment and I will try using a different computer so that the serial port hardware is (presumably) different. If I can't solve it, I will indeed create an issue on github as Mark suggests.

Cheers

Peter Allan


On 5 December 2017 at 19:30, Mark Pizzolato <Mark at infocomm.com<mailto:Mark at infocomm.com>> wrote:
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<mailto: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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