[Simh] Problem using a real VT terminal with simh

[Peter Allan]

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.

The host for simh is a standard PC running CentOS 6.9.

I am connecting the VT420 to the PC using the 25-pin RS232 connector at the
back of the VT420, going via a 25-pin to 9-pin adaptor, to the 9-pin RS232
connector on the PC. I am convinced that the physical connection is correct
since I can start an agetty process at boot time and log into CentOS from
the VT420. Everything works and displays as I would expect.

For what follows, I have removed the starting of the agetty process so that
nothing is controlling the /dev/ttyS0 device before I start simh.

The simulated microVAX 3900 is running VMS 7.3 from an RA92 disk. This
configuration has been running successfully for a long time and I am only
now trying to add the connection of a physical terminal.

In the vax.ini file, I have added the lines

SET DZ LINES=4
ATTACH DZ LINE=0,CONNECT=SER0

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.

Furthermore, when I log into the microVAX, the VT420 goes through a reset,
as though I had turned it off and on. I think this is because the login
sequence does a SET TERMINAL/INQUIRE since if I type that command, exactly
the same thing happens. This seems particularly odd since I have used that
command many times with a real computer (although admittedly not with this
particular terminal) and I have never seen this happen before.

The VT420's RS232 port is set to a speed of 9600, with 8 bits and no parity
and the TTA0: device on the microVAX also gets set to this combination.

Hopefully I am just doing something wrong, but I can't see what that might
be. Can anyone help with this please?

Peter Allan
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