Re; What to look for in a Model 33 Teletype - a newb's buyer's guide?

tony duell ard at
Fri Nov 21 15:28:48 CST 2014

> >
> > I suspect that most, if not all, of those RS232-TTL converters are just
> > a MAX232 + the necessary capacitors, or something very similar.
> >
> The one I linked to seems to actually use an Atmel microcontroller that's
> emulating the MAX232.. else I don't know why there would need to be a uC
> chip on the board.
> I mean to say, the MAX232 is effectively a one-chip 232 to TTL converter
> solution, eh? Why would they need a micro to supplement it?

Possibly data format or baud rate translation. Or perhaps the TTL interface is not
simply the asynchronous serial signals, but something like SPI. In any case you don't
want it here.


> Now about the optos.. do we really +need+ to use the HCPL 4100/4200 for
> this application? Wouldn't other, more generic optos serve the same roles?
> I realize that the 4100/4200 are high-performance parts, designed to handle
> relatively high data rates - but at only 110 baud, I'd think just about any
> opto with adequate light / dark resistances would be OK.
> Then again, I've never played with this stuff.. might be completely wrong.
> But I ask, as I already have a bunch of optos sitting in a drawer. ;-)

Classic computers with current loop interfaces often opto-isolated them using
'any old optoisolator' As you said, it's only 110 baud, so speed is not an issue

The current-loop -> RS232 side is quite easy, with 20mA available just about any 
optoisolator will work (you may even need to shunt some of the current round the 
LED, check the maximum If of the LED in the device you are using).

The RS232 -> current loop side might be a little more work, in some cases the
optoisolator's phototransistor won't pass 20mA. You may need to add an 
external transistor (essentially a darlington pair circuit). 


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