AT&T physical terminal with RC2014

[James Harland]

Hi everyone,

I have an RC2014 Classic ][. I was over the moon when someone who was selling an Epson HX-20 for dirt cheap on the Kazakhstan equivalent of Ebay also offered an AT&T dumb terminal for sale, also very cheaply, so I snapped it up. It switches on, but from the settings menu, it seems it doesn't support 115200 baud, the options are 300, 1200, 2400, 4800, 9600 and 19200 baud. Is there anything I can do to get my RC2014 talking to this terminal (ideally a quick fix, though I am ready to hear I need another I/O board).

The other question I have is about connector wires. The monitor of the terminal has two 25 pin female (I'm guessing serial) ports. I'd also like to connect the terminal to one of my Linux boxes while I'm working on doing so with the RC2014.

Here are some photos:

[James Harland]

OK I realize I had things set to "printer" - when I set things to "host" I can get up to 38400 baud, but still far off 115200. 

Just a thought - I seem to remember the Orton 3C (which is on my Christmas wishlist) has a 9600 baud connection?

Another thing which is more specific to the terminal - it is only visible (and pretty dim) when at maximum brightness. For about 80% of the brightness dial it is completely invisible. Does anyone know what the issue might be and what might need fixing?

[Michael Druckenmiller Sr]

First a caveat or two. I never got around to completing my 2014-Pro. I do have a couple TS2068's

The terminal could have bad phosphor burn or low voltage out of the HV Power supply. As you didn't mention dark spaces on the screens where characters should be, I would assume the it is the HV Power Supply.

In my case my thermal printer died and I was aggravated by the slow printout, so I designed and built a M6850 Serial interface, and redirected Print # to some Assembly code.

I have not rewritten the Terminal Code for the PC to run on my Linux Boxes. I could not get any 'stock' Terminal apps to work as all were too 'smart' expect a certain protocol and handshaking.

I suspect your used Terminal is emulating a specific Terminal such as a VT100.

At any rate a Z80 can run data at 115200 without any protocol overhead.

I would be happy to provide my schematics and code if interested.

I haven't done much with the RC-2014 because I had a few heart attacks after getting it, and I have working TS2068's to play with. Still on my to do list. BTW I prefer the simplicity of the M6850 over the more complex Z80 serial peripherals.

Mike Sr.

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[Fernando Carolo]

Hi James,

Looking at the pictures, it looks like the terminal uses RS-232 for communication, based on the DB-25 connectors. This means "real" RS-232, with +12V/-12V signals, and you have already seen that it can go up to 38400 bps. This means you'll need a way to physically connect it to your Classic II and to get them working at the same speed.

You can't connect this terminal directly to an RC2014 Classic II that uses 0/5V (a.k.a. TTL levels) for its serial port. You will probably kill your Classic if you try. You need an "RS-232 to TTL level adapter", there are a few of those available from online sellers. I did a quick search and most of them come with a 9-pin port, technically a DE-9 although we usually just call them DB-9 anyway. I'll also need a DB-25 to DB-9 (ahem) cable in a null-modem configuration.

The other tricky part is how to make the Classic II talk at 38400 bps. I am not familiar enough with it and the docs seem to imply that the serial I/O module will only work at 115200. I'll have to defer this to someone more familiar with it.

Best,

Fernando

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[Michael Druckenmiller Sr]

I used the MAX232 myself, and I believe there is such a module in the RC2014 page, somewhere.

This article may help: https://bryceautomation.com/index.php/2023/04/11/using-rs232-on-the-rc2014/

Looking for my schematics, etc.

Mike Sr.

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[Spencer Owen]

Hi James,

As others have alluded to, there are two issues to be overcome to get your RC2014 to talk to your terminal; Speed and voltage levels.

The voltage levels are fairly easy. The terminal will be running at full fat RS232, which is typically +/-12v (could be anything from +/-9v to +/-18v) and is inverted polarity compared to the 5v TTL serial on the RC2014. This is easily converted with a MAX232 chip and 5 capacitors.  I used to offer an upgrade for the original RC2014 Classic, but pre-assembled modules are so cheap on eBay it is just easier to get one of those. Typically these come with a 9 way D socket, so you'll need a suitable cable to adapt to your 25 way socket.

Regarding the speed, however, there are a few options, although none of them are ideal. The 115200 baud of the RC2014 is derived from the 7.3728MHz clock.  Slow the clock down and the baud rate drops.  So if you swapped the crystal for 2.4576MHz, you'd be getting 38400 baud out of your serial port. Alternatively, dividing the existing clock down would have a similar effect.  Dividing by 3 isn't straight forward, but dividing by 4 with a single 74HCT393 would give you 28800 baud. Replacing your current clock module with the Dual Clock Module (https://rc2014.co.uk/modules/dual-clock-module/)  will allow you to do this easily.

Slowing down the clock doesn't just change the baud rate though. It will also drop your CPU speed by 3 (or 4) too. Cutting the incoming clock track in to the serial module will allow you to feed that from Clock 2 of the Dual Clock Module. ie Clock 1 supplies 7.3728MHz to the CPU just as nature intended, and Clock 2 could supply 2.4576MHz to the UART.

And at the risk of dashing your dreams on the Orton 3C - Yes, technically this is capable of running 9600 baud serial, however, for just a serial echo program you need to toggle in 197 bytes of code first. Currently I am not aware of any practical way to program the Orton 3C via the serial port. (The 3CX which PeterO has posted about on here is highly modified and has a lot of custom code written for it.)

Hope that helps

Spencer

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[Andrew Carr]

Something else of note/caution for the MAX232 chips. They're one of the more counterfeited chips out there. I've found that it's better to buy from a supply house (places like mouser, digikey, etc in the US) than to roll the dice on EBay/aliexpress, or Amazon. 

If you do want to risk it, find the modules with the highest chip count. Like Spencer alluded to, it needs capacitors to run properly and the fakes are more likely to leave them off. The "quick" way to see if it's a bum chip is to hook it to power and touch the IC. If it burns you, it's bad. Warm to the touch should be okay but not being able to leave your finger on the chip is a pretty solid sign. 

Oh, and a Homer Simpson "do'h!" that I learned the long way - you need to feed it +5V and have the TX/RX lines go straight through, don't swap the lines. (or is it the other way around? Arg, it's a bit too early in the morning...)

Good luck with the terminal! Got a Wyse myself that's sitting in the project pile....

And if you haven't found it, checkout the text terminal wiki: https://terminals-wiki.org/wiki/index.php/Category:IBM 

Andrew

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[James Harland]

Thanks everyone - I see I have got myself deep into an area I was blissfully unaware of the complexity of before :-D. I had a feeling that serial on the Orton would involve a lot of flicking for little result, but I'm still game to try it. I guess there it would still need the MAX232 - I need to look into that, thanks for the caveats Andrew!

I think my gameplan is to go from the simple to the more complex, starting with getting Linux to talk to the terminal, then something 8 bit that wants to do RS232 (the Epson HX-20 for example if I can get it working), then see about the modfications needed to get the RC2014 and/or the Orton connected.

The HX-20 is another project, I have a German one which I picked up in Berlin for 10 Euros about 20 years ago, which worked until 2020, and now the one I have bought here, which may or may not work - I need to source a suitable power supply.  If it also doesn't work I have a feeling that with two of them I have better chances of resurrecting one of them.

I seem to remember the BBC Micro did RS232 natively, but I can't remember what the deal is with the ZX Spectrum - do you need an Interface 1? I have a Speccy but no Beeb.

[Steve Cousins]

You could use a module with a built in baud rate generator. You'll still need to TTL to RS232 converter.

[Michael Druckenmiller Sr]

Wow, almost identical to what I did for the TS2068! The MAX232 is still the best course of action.

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[Michael Druckenmiller Sr]

I am offering this chagrin so some of you others won't do the same. :) While it was for the TS2068 it is still Z80 stuff and my 'thought I knew it all' may help another. One day I'll get my RC2014 built...

On 11/11/25 11:05 AM, Steve Cousins wrote:

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[Steve Cousins]

Good account of "How NOT to Design a serial Port". We all have our bad days!

[James Harland]

I will at some point get the dual clock card and do this for real, but for now I can have the feel of the RC-2014 on my AT&T terminal... via minicom via the attached Linux box. But who's to know eh? ;-)