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Comment by 8bitsrule | Hacker News Reader

Here's a little more about 20ma: https://www.electronics-notes.com/articles/connectivity/seri...

It mentions a couple things worth noting: for 20ma loops, depending on distance, speeds over 19.2kb might not work. It also mentions RS-422 [1] , RS-485 (both up to 10 Mb/s) as options. (Both can be full-duplex.)

It also mentions a part (rarely mentioned these days) called an 'opto-isolator'. Might be an addition worth considering ... fun toys that let you keep the smoke-testing on the cheaper side of the project.

[1] https://www.electronics-notes.com/articles/connectivity/seri...

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20ma for an ASR-33 is easy. You're just driving a transistor, not the selector magnet itself. The ASR-33 has its own magnet driver.

The older teletypes, the Model 14 (1920s) and Model 15 (1930s), take more driving. They need 60mA at 120V DC, because you're operating the selector magnet directly. The usual setup is a 120VDC supply with a 10K 2W resistor in series, switched with an opto-isolator. That is simple and works fine.

I designed and built a driver for the older machines that's powered entirely from a USB port, and converts 5V to 120V.[1] This was a fun electrical engineering project. Had to design an unusual switching power supply. A few other people have built this board, including some from the Computer Museum in Mountain View. That packages up the whole power problem in a nice little box. Requires surface mount soldering, though.

[1] https://github.com/John-Nagle/ttyloopdriver

Sorry, 2K, 10W. I use thick-film vertical resistors, so as not to scorch the PC board.

When I was a kid, my dad connected a Model 19 teletype to our Apple II computer for amateur radio teletype (RTTY). He used an opto-isolator to protect the Apple II from the 60 ma loop that the Model 19 used.

0-20mA or 4-20 mA loops are super common in industrial automation for analog sensors (pressure, temperature, etc) as they're very resistant to electrical noise unlike those using voltage. The nice part is they can be easily converted back to voltage at the point of terminus using a resistor.

Opto-isolators are an ideal way to interface to 20mA TTY IMHO. Keeps the old stuff away from the new stuff.

Open collector transistor connected by light only. Love it.

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