20 May
2001
20 May
'01
9:28 p.m.
Hi all.
During the weekend I found some time to check the VT102 -> LA210 connection.
The suggestion Tony made, was very good. Stupid that I didn't think of it.
I connected a VT220 to the VT102 printer port using the BC22D cable.
Then I sent characters to the VT220. The letter E showed up as a zero and
some other characters. After that nothing appeared on the VT220 screen.
It was locked up.
Next I connected my Tek 465 to the transmit pin (#2) of the printer port
on the VT102. When I do the printer port test (screen full of E's) and
send that to the printer port it became obvious why the LA210 prints
garbage.
Pin #2 is rougly -19 Volts.
When characters are sent there are only spikes visible with an amplitude
of about 20 V. superimposed on the -19 V.
I hope I did not damage the LA210 as the transmit signal with the spikes
go to almost -40 V.
Or am I making a measurement error?
The pin #2 (transmit) on the VT102 printer port is only connected to the
(high impedance) input of the oscilloscope. The ground of the Tek is
connected to pin #7.
Do I need a resistor between pin #2 and ground (#7)?
Henk.
-----Original Message-----
From: ard(a)p850ug1.demon.co.uk [mailto:ard@p850ug1.demon.co.uk]
Sent: donderdag 17 mei 2001 21:09
To: classiccmp(a)classiccmp.org
Subject: Re: LA210 problem with the control lines
Hi all.
Yesterday I connected my LA210 Letterprinter to the
printerport of a VT102.
Both are set at 9600 Bd, 8 databits, no parity and 1 stopbit.
However when I send some 5 characters from the VT102 to the
printer, it prints many mirrored question marks and
other 'strange' characters.
Do you _know_ that the printer port on the VT102 is working correctly
(and at the right baud rate)? Try connecting another terminal
(or a PC running a terminal emulator program) in place of the LA210
and see what happens.
In many systems, the transmit and receive baudrates are
derived from the same crystal. and in some cases, particularly
printer ports, DEC used the same baud rate generator for the transmit
and receive clocks.
That means, of course, that you can;t select different transmit and
receive speeds, but that's rarely needed for a printer port.
Now a loopback test checks the port against itself. So, particularly
in the latter case, it could be running at totally the wrong
baud rate and still pass the loopback test. You want to check against
something else, like a PC.
Okay, I thought, the baudrate setting, etc. is
not correct.
But it is.
These are the checks I have done so far.
1) The used cable is an original DEC BC22D-25. This cable
is mentioned in the
LA210 Letterprinter User Guide.
It's printing something, so the data leads must be the right way round.
And the handshake (control) lines are not important until you
can get the printer to print some characters correctly.
I do not understand the message "Control
lines failed".
It's a problem with what are normally called the handshake
lines (RTS, CTS, etc). Either you've not got the right loopback plug
wiring, or you've got a problem on the LA210 logic board.
I have no field maintenance print set of the
LA210 ....
Nor do I. I do have the LA100 printset somewhere which is
electrically very similar, though.
-tony