[GreenKeys] A reliable current-loop-to-RS232 converter

[Gabriel Egan]

I'm trying and failing to get my new ASR-33
communicate with my Altair 8800 clone via
a current-loop-to-RS232 converter purchased
from the usually reliable UK retailer RS
Components.

The converter has rather a lot options, and
it's able to support Active as well as Passive
current-loop operation. I've got it set to
Active and it's got 5v coming in from a USB
cable.

On the Teletype end, I have four coloured wires
coming from the Teletype:

TTY Send: Red (3) and Black (4) wires (not
otherwise differentiated)

TTY Receive: White (7) (positive) and
Blue (6) (negative)

(These assignments including the numbers 3, 4, 6, and
7 come from a label on the data cable coming from the
Teletype).

On the converter I have a terminal block with
five screws. If I select Full Duplex operation
and select that both Tx (Transmit) and Rx (Receive)
are Active then those five terminals have these
assignments:

TB1: Tx I+ Out / V+
TB2: No Connection
TB3: Rx I+ Out / V+
TB4: No Connection
TB5: Tx/Rx I- In / V-

(This is exactly how the manual describes them;
I assume that "I" means "current", as Andre-Marie
Ampere himself decreed.)

On the principle that what the converter considers
to be the act of 'sending' is what the Teletype
considers to be the act of 'receiving' I connected
the White wire from the Teletype to TB1 and the Blue
wire from the Teletype to TB5. I set the converter
to DCE operation and booted the Altair: Success! The
MITS BASIC welcome message prints on the Teletype
and I'm asked to enter Memory Size, just as
expected.

So, just the other two wires to connect and all
should be well. This is where I get stuck!

I connected the Red wire from the Teletype to the
TB3 and the White wire from the Teletype to
TB5. No luck. I reversed them: White to TB5
and Red to TB3. No luck. Then I tried all
permutations of the remaining terminals
(everything except TB1 and TB5 since these
are clearly in use for data going the other
way). No luck.

Then I did all the above again with the converter
set to Active on the Tx side and Passive on the
Rx side. (No use setting the Tx side to passive
as this just makes the Teletype 'run open'.)

What else might I change on the converter? 60ma
instead of 20ms? No luck (Teletype just 'runs
open'). Half Duplex instead of Full Duplex
(No good: seems to be for systems sharing the
same pair of wires for both sending and receiving,
and we know we're not doing that.)

I think I've run out of options with this particular
converter. Is a 5v USB supply not enough current?
This seems unlikely since they sell this converter
for current-loop applications.

If anybody on the Greenkeys lists can see my fatal
error(s) in the above, I'd be grateful to know.

Alternatively, can anybody point me towards the
seller of a current-loop-to-RS232 converter that
is known to work with Teletypes and for which
they can tell me exactly where my four coloured
wires go? If anybody is willing to make me such
a converter, I'd be happy to pay any reasonable
price.

Regards

Gabriel Egan