Hi all --
I'm attempting to get my 8/L talking to the outside world. It's
outfitted with the typical Teletype interface, which I'm attempting to
connect to a PC via acurrent loop to RS232 adapter.
Specifically, I've got one of these:
http://www.bb-elec.com/Products/Serial-Connectivity/Serial-Converters/Curre….
At the moment I have it configured (per the manual) assuming the 8/L is
an active loop device (i.e. the converter's transmit lines connect to
the 8/L's receive lines, and the 8/L's transmit lines connect to the
converter's receive lines). With this configuration I'm able to get the
8/L to send data to the PC successfully.
That is normally the more difficult way to get working...
I'm not having much luck getting the8/L to receive anything from the PC,
however. If I put my multimeter in the loop, I see a current of about
40mA, which drops to between 30 and 37mA when the PC is sending data
(depending on what data's being sent). However, I'm not getting much of
anything onpin "E" of the W076 (Teletype Connector) board -- this is
supposed to be the "Logic Output" and I'm assuming that should be the
TTL-level data signal, correct?
I don't know this machine, but I've worekd on DEC current loop serial
interfces in PDP11s and the PDP8/e. I am going to assuem this was
originally intented ot link to a Teletype, a model 33 or something.
The transmitter side of a Teeltype model 33 is entirely passive. It's
just switch contacts. The idle state is 'closed' -- that is the 2
connections are shorted together by a pretty low resistance. THis opens
(to essentially infinite resistance) for each 'space' bit in the data stream.
Which means you should be able to trac signals by connecting a simple
switch to the 2 wires on the transmit loop of the PDP8/L in place of this
interface box, opening and closign the swtich, and seeing what, if
anything, changes state. Once you can get the right TTL levels at the
right point with that, you can then try again with the covverter.
I know nothign about this coverter. I assume the output is electrically
isolated from ground (and from the PC side). The DEC current loop input
is not isolated from ground, and if the output of the converter isn't
either this could cause all sorts of problems.
-tony