LA210 problem with the control lines

From: Gooijen H <GOOI_at_oce.nl>
Date: Mon May 21 04:28:10 2001

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_at_p850ug1.demon.co.uk [mailto:ard_at_p850ug1.demon.co.uk]
> Sent: donderdag 17 mei 2001 21:09
> To: classiccmp_at_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
Received on Mon May 21 2001 - 04:28:10 BST