RS-232 to current loop (again)

General discussions related to the Altair 8800 Clone

RS-232 to current loop (again)

Postby mail@gabrielegan.com » October 9th, 2015, 3:48 am

I'm still trying to hook up my ASR-33 to my Altair clone. I've got a new current loop connect for B&B Electronics and they say it's ideal for this application. Trouble is knowing what to connect to what. On the cable from the ASR-33 I have:

Red wire TTY send
Black wire TTY send
White wire TTY receive positive
Blue wire TTY receive negative

On the converter I have on the left side terminals for:

TD
RD
10-30VDC
GND

and on the converter I have on the right side terminals for:

T+
T-
R+
R-
Current GND
Current GND

I've connected it like this:

Red wire to T+
Black write to T-
White wire to R+
Blue wire to R-
Pin 2 from Altair serial port to TD
Pin 3 from Altair serial port to RD
Unsheathed GND from Altair serial port to GND
Positive from a 20VDC power supply to 10-30VDC
Negative from a 20VDC power supply to GND (NOT Current GND)

Wired like this, the ASR-33 just chatters away cycling the print hammer but not printing anything (like it does when it has no current loop connection). I've also tried connecting Pin 7 from the Altair serial port to GND on the RS-232 side of the converter (instead of the unsheathed GND from the Altair to GND on the RS-232 side of the converter) and connecting the negative side of the 20VDC power supply to Current GND on the current-loop side of the converter instead of connecting it to GND on the RS-232 side of the converter (and I've done the permutations of this).

All to no avail. Can anyone see what I'm doing wrong?

Gabriel Egan
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Re: RS-232 to current loop (again)

Postby AltairClone » October 9th, 2015, 8:35 pm

Post the model number of the b&b converter you're using, or a link to the manual.

Mike
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Re: RS-232 to current loop (again)

Postby AltairClone » October 10th, 2015, 9:14 am

I see it connecting as shown here:
Attachments
image.jpg
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Re: RS-232 to current loop (again)

Postby mail@gabrielegan.com » October 12th, 2015, 11:35 am

You're a ruddy marvel, Mike! I would never have figured that out in a million years, although the assignments make sense now you point them out. All working fine now! Much appreciated. In case anyone wants to see our two Teletypes running Multi-user BASIC on the Altair, there's a video at http://gabrielegan.com/scratch/Altair/Teletypes.mp4.

Gabriel Egan
Minimal Computing Lab
De Montfort University
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Re: RS-232 to current loop (again)

Postby mail@gabrielegan.com » August 3rd, 2017, 12:32 pm

My RS232-to-current-loop converter (used to connect my ASR33 Teletype to my Altair clone) just died and the new converter has different connections from the old one. The old one didn't use T+ and R+, just T- and R- and a pair of Grounds. The new one has T+, T-, R+, and R-. The four wires from the ASR33 come from terminals 3, 4, 6, and 7 on the terminal strip inside of the ASR33. Does anyone know the mapping between those terminals and T+, T-, R+, and R-? Trying to infer it from the connections on the old converter doesn't work: the ASR33 just 'runs open', as they say. I have Mike's schematics for a current loop converter, and although this usefully has the terminal strip numbers it uses the conventions of "Data to Teletype +", "Data to Teletype -", "Date from Teletype +", and "Data from Teletype -". I don't know if "T[ransmit]" and "R[eceive]" are meant to take the perspective of the Teletype or of the converter. That is, "T" could mean "Transmit this signal from the computer to the Teletype" (in which case this connection should go to "Data to Teletype" or "Transmit this signal from the Teletype to the computer" (in which case this connection should go to "Data from Teletype"). Mike's conventions are obviously better, if only everyone used them!

Regards

Gabriel
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Posts: 78
Joined: October 11th, 2014, 8:12 am

Re: RS-232 to current loop (again)

Postby AltairClone » August 4th, 2017, 11:08 am

Gabriel,

Post a link to a manual for the new unit and we can figure it out.

Mike
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Re: RS-232 to current loop (again)

Postby mail@gabrielegan.com » August 7th, 2017, 4:04 am

Dear Mike

I bought the new converter secondhand on Ebay so there's no manual, but there is a cheat-sheet for the connexions, which I've put a picture of at:

http://gabrielegan.com/scratch/RS232-pic.pdf

If you can make any sense of how the 6 jumper switches should be set for an ASR33, that'd be even more dandy!

Regards

Gabriel
mail@gabrielegan.com
 
Posts: 78
Joined: October 11th, 2014, 8:12 am

Re: RS-232 to current loop (again)

Postby AltairClone » August 9th, 2017, 9:18 am

I'm not sure what to tell you for sure on this one without seeing a schematic. A Teletype provides no current for either the transmit or receive loop, so power for the loop must come from the outside (e.g., your RS-232 to current loop converter). I see a jumper option for providing power for the transmit loop, but there is not the same option for the receive loop, so this converter may not work for you.

I would assume T+ and T- pins are data out from the converter and data in to the Teletype. Similarly, I'd assume the R+ and R- pins are data in to the converter, data out from the Teletype.

I would apply 12 volts to the loop power input and set the transmit jumpers to "Active Transmit." Set W3-W4 with the "Data Transmit" setting and W5-W6 with the "Data Receive" setting. At this point, the constant chatter on the Teletype should stop and data you send to the Teletype should print. However, I'm not sure that data you type on the Teletype will make it back to your computer unless choosing the "Active Transmit" option and applying power also powers the receive loop.

Keep us posted!

Mike
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Re: RS-232 to current loop (again)

Postby mail@gabrielegan.com » August 9th, 2017, 12:19 pm

Dear Mike

I see the problem regarding "12v from outside for active transmit" -- I'd assumed they meant both transmit and receive but perhaps not. I will try the settings you suggest and see what happens.

I don't suppose you have someone at your place (a trainee engineer perhaps?) whom I could pay to make me one of those nifty fits-under-the-hood converters that you designed? I'd pay top dollar for that solution.

Regards

Gabriel
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Posts: 78
Joined: October 11th, 2014, 8:12 am

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