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

Thu Aug 13 11:55:33 EDT 2015

Correct!  Except the 33 does not supply any loop voltage in Line mode in its normal configuration.  

Wayne 

----- Original Message -----

From: "Jack" <wa2hwj at att.net> 
To: "Gabriel Egan" <mail at gabrielegan.com>, greenkeys at mailman.qth.net 
Sent: Wednesday, August 12, 2015 8:42:15 PM 
Subject: Re: [GreenKeys] A reliable current-loop-to-RS232 converter 

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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