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

Sat Aug 15 15:32:33 EDT 2015

Gabriel,

I think what maybe has been missed so far in this thread is that any configuration change you make to the interface must also be reflected in the
configuration of the Teletype.  On the Model 33, at the terminal strip in the back of the UCC, there's a very specific way these wires should be used.
My Model 33 came to me configured for Half Duplex, 60mA.  That had to be changed to Full Duplex, 20mA.  This is all laid out in the manuals, and
there's even some great websites out there with knowledge about how to interface teletypes to computers.

The thing about using 3 wires instead of 4 wires for the actual loops sounds quite a lot like half duplex configuration to me, which you should not be
using with your Altair.

When I first got my 33 running, I needed a way to get tape data off the internet and onto tape... so I quickly whipped together this interface:
https://www.circuitlab.com/circuit/mnartk/screenshot/1024x768/

To make the 12v, I simply used a full-wave bridge rectifier built out of a handful of 1N4001's and a small electrolytic to smooth it all out.

Since I didn't have any 1/4 watt resistors at around 600 ohms, I used a pair of 1/8 watt 1.2k resistors in each loop for the current limitation.

I built the whole thing up on a solderless breadboard, and used it for a couple weeks until I finally broke down and bought a BlackBox CL050
interface for about $20 on ebay, which works perfectly when configured correctly (the manual is still on their website, along with schematic).

On the circuit I've outlined above - one thing to keep in mind is that on the 232 RX side, it will not drop below 0v for obvious reasons (i.e. will not
go to -12v).  Most 232 interfaces are fine with this, and this circuit does work with TTL level 232 signals.

Here's a photo of that circuit on breadboard: http://i.imgur.com/DIKYPez.jpg

There's a lot of ways that CL can be configured.  Making sure what you have configured on the interface is mandatory to match what you have the 33
configured for.

Hope this helps! :)

cheers,
j

Gabriel Egan(mail) 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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