Serial port settings for a real Teletype

General discussions related to the Altair 8800 Clone

Re: Serial port settings for a real Teletype

Postby AltairClone » July 9th, 2019, 2:29 pm

Ken,

Yes, I still have the adapter boards for the '33 available.

The Altair Clone serial ports are wired as DCE. That means data out from the Clone to the TTY is on pin 3, and data from the TTY to the Clone is on pin 2. Signal ground is pin 7. See page 2 of the Altair Clone manual for more information.

Mike
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Re: Serial port settings for a real Teletype

Postby kwiebe » July 16th, 2019, 6:27 pm

AltairClone wrote:Ken,

Yes, I still have the adapter boards for the '33 available.

The Altair Clone serial ports are wired as DCE. That means data out from the Clone to the TTY is on pin 3, and data from the TTY to the Clone is on pin 2. Signal ground is pin 7. See page 2 of the Altair Clone manual for more information.

Mike

Thanks, Mike.

Sometimes I think I'm in documentation overload...I guess that's preferable to the alternative.

Anyway I appreciate the info and pointers, as always. And thanks for sending out the adapter!
-Ken
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Re: Serial port settings for a real Teletype

Postby kwiebe » August 8th, 2019, 7:24 pm

Mike,

Ran into unexpected result connecting 33 to Altair.

I followed your modular DB-25 DCE instructions and ran SERBUF after configuring port 2 per those instructions. I didn't get data on either TTY or clone so I inserted an RS-232 breakout box into the connection.

I saw the clone transmitting on pin 3 as expected but the TTY was also transmitting on pin 3. So even though it went against your instructions, I made a DTE version of the modular connector and tried again.

After doing that, I got data in both directions. The TTY printed the characters I typed on the terminal. But typing on the TTY resulted in random characters on the terminal screen.

The tendency would be to blame the TTY but I have connected it to a dumb terminal and successfully got two-way comms (using all the same parts).

Wondered if you might have any ideas. Also a thought I had about the first issue, where I had to rewire pins 2/3, do your instructions assume a straight-thru phone cable or a "rolled" one? Mine is straight-thru but I got to thinking if your instructions assume a rolled cable then that may explain why I would have to swap 2/3.

EDIT: I just read Gabriel's post upthread -- d'oh -- thanks, Gabriel! (cross-wired cable) -- but I still have the one-way comm problem

Thanks
-Ken
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Re: Serial port settings for a real Teletype

Postby AltairClone » August 8th, 2019, 7:33 pm

My instructions assume a standard telephone cable which ends up swapping all pins. For standard phone wiring, given a flat cable, the hook is on the same side of the cable at both ends.

Mike
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Re: Serial port settings for a real Teletype

Postby kwiebe » August 8th, 2019, 8:03 pm

Thanks. I somehow found a standard phone cable and went back to the DCE wiring. That now works as expected. But I still don't get the correct characters on the terminal side. But correction to what I said earlier - the characters are not random they are just not correct.

So, I always get the same wrong character on the terminal side each time I press a given character on the TTY.

This seems like a big clue but I'm not sure what it points to.

Again, I cannot duplicate this when connecting the TTY to a dumb terminal directly.
-Ken
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Re: Serial port settings for a real Teletype

Postby AltairClone » August 9th, 2019, 10:48 am

Let me make sure I understand your setups correctly.

1) If you connect the TTY to a dumb terminal, what you type on the terminal keyboard prints properly on the TTY and what you type on the TTY shows up properly on the terminal screen.

2) In your second configuration, you have the TTY connected to the 2nd serial port on the Altair Clone and run the "SERBUF 2" command under CP/M. On the CP/M operator console, what you type properly prints on the TTY, but when you type on the TTY, the wrong character displays on the operator console. However, the same wrong character displays each time a given character is typed on the TTY.

In configuration #2, what is the operator console - a terminal emulator or the same dumb terminal from configuration #1? If the operator console is not the dumb terminal, what happens if the dumb terminal is used as the CP/M console?

In configuration #2, give several examples of the character typed versus the character displayed.

Mike
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