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

[Jarratt RMA]

I bought a Westermo converter which has worked well for me, there was one
listed on ebay not too long ago, and I seem to remember it was in the UK. I
am not at home now to check how I set it up, and I may not get time to look
for a while.

Regards

Rob

On 12 August 2015 at 15:32, Gabriel Egan <mail at gabrielegan.com> wrote:

> 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
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