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

[drlegendre .]

FYI, I can't parse this .docx file you sent.. it just comes up as a blank
page.

Using Libre Office, here.

On Wed, Aug 12, 2015 at 7:42 PM, Jack <wa2hwj at att.net> wrote:

> Gabriel,
>
> Here is a very simple RS232 to loop circuit. You tie the 33's keyboard and
> selector driver leads in series and hook them to the LOOP input. You do
> need
> to provide external loop voltage (maybe from the 33).
>
> I had PC boards for this circuit, but they are all gone.
>
> Jack
>
> By the way...verify the 33 keyboard's leads by testing for a short
> circuit with an ohmmeter. As you type on the keyboard, you
> will see the ohmmeter follow the typing. Then, logically, the
> other two leads are the selector magnet driver. You can test
> them with a 9V battery to see if the machine runs closed.
>
>
>
>
> -----Original Message-----
> From: GreenKeys [mailto:greenkeys-bounces at mailman.qth.net] On Behalf Of
> Gabriel Egan
> Sent: Wednesday, August 12, 2015 9:33 AM
> To: greenkeys at mailman.qth.net
> Subject: [GreenKeys] A reliable current-loop-to-RS232 converter
>
> 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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