RS-232 to current loop (again)

General discussions related to the Altair 8800 Clone

Re: RS-232 to current loop (again)

Postby mail@gabrielegan.com » September 21st, 2017, 7:02 am

Dear Mike

I'm connecting the Teletype to the Altair clone, not a PC. I can check the 'handshaking' setting, but isn't that ruled out as a possible cause by the facts that i) a different Teletype (a 35KSR) attached to the same Altair port works fine, and ii) this Teletype attached to the same Altair port and running on my old RS232-current-loop-adapter works fine for a few hours before the adapters starts to misbehave (which is why I'm trying to use a new adapter)?

Regards

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

Postby AltairClone » September 21st, 2017, 11:22 am

If handshaking is enabled in the Clone, then the other configurations may work because those adapters are asserting RTS out to the Clone (becomes CTS in to the Clone) and maybe this newest adapter doesn't. So yes, it's a possibility.

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

Postby mail@gabrielegan.com » September 22nd, 2017, 4:25 am

Dear Mike

It's always an education to converse with you: I see what you mean about different adapters behaving differently regarding handshaking and will try changing that setting on the clone.

Regards

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

Postby mail@gabrielegan.com » September 30th, 2017, 5:58 am

Ah well, still no good: even with RTS and CTS constantly asserted in the Altair, my new RS-Components current-loop converter lets the Altair send data to the ASR-33 Teletype but doesn't let the Teletype respond. Weird one. But my old B+B converter still works at least for an hour or two. I guess the work-around is just to power off the converter every couple of hours, at least until I can find another B+B converter of exactly the same design second-hand on Ebay.

Regards

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

Postby AltairClone » September 30th, 2017, 9:15 am

Looking at the settings sheet you posted earlier, I think I know what the problem is.

The converter regulates the 9v power input down to 5v. The receive opto uses the sending device (in this case, the Teletype's output to the converter) to provide a low impedance path to ground which allows current to flow from the 5v supply, through a resistor, through the LED in the opto, to then produce a logic transition on the output side of the opto.

However, the output of the Teletype is not a low impedance output. Between the knife-edge/wire contacts on the keyboard and the carbon wiper on the distributor, resistance in the 10's of ohms is not uncommon. This, in turn, means much less current flows through the opto LED than designed for, so the output of the opto is not triggered.

The original design of the 88-SIO board used 5v to read from the Teletype, but was later changed to over 30v (+16v to -16v) to improve reliability. 5v can be used with a different circuit design than your converter uses, but the teletype output side has too much resistance to work reliably with your converter's design.

If you can wait, I am committing to making the RS-232 adapter that simply plugs onto the J2 connector this fall/winter. No wiring, no screw terminals, no AC adapters required - just plug it in and go.

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

Postby AltairClone » September 30th, 2017, 11:12 pm

Try this as an experiment and see if you can get receive working. Use a NEW 9 volt battery, set the receive side to passive (PSV), and change the receive wiring as shown. Transmit data to the teletype is unchanged (still ACT).

You can use a 12 volt supply and a 470 ohm resistor instead if needed.

Mike

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Re: RS-232 to current loop (again)

Postby mail@gabrielegan.com » October 1st, 2017, 1:34 pm

Mike, I'd be most interested in the plug-in solution to my ongoing current-loop converter woes -- I can wait! I'll try the 9v battery fix on Wednesday when I'm next in the lab (and after my Amazon-sourced supply of resistors and 9v battery connectors has arrived!)

Regards

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

Postby mail@gabrielegan.com » October 5th, 2017, 6:40 am

Alas, still no good! (Same fault: TTY can receive from Altair but not transmit to Altair.) I guess I'll bump along with my B+B converter until you've done that super-duper fit+forget solution, Mike. Thanks for the attempts -- seems like the new adapter just isn't behaving right.

Gabriel
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