Improved Apple Teletype Emulation

A previous post showed how I attempted to connect the SCELBI TTY to an Apple IIe with an old serial card that supported current loop. Well I’ve been working on some of the issues and now have it working good enough to post some of the details. With the standard Apple 80 column scroll routine, I can only reliably get 8 lines to scroll at 110 baud, but it’s working pretty decently, with that limitation.

First of all, here is how the TTY card is connected. The standard SCELBI MEA software assumes these port assignments.

Apple Serial To SCELBI TTY Connection Diagram

Apple Serial To SCELBI TTY Connection Diagram

If you try repeating this experiment, be aware that there are two versions of the Apple Serial Card firmware,. However, since I don’t use the standard driver, it doesn’t matter which version you have.

The simple TTY emulation source code can be downloaded from It is written to use an assembler called DASM, which can be found and downloaded with a web search.

You can experiment with changing the window size by changing the second line in the program. Note that I started with a “hijacked” version of the original serial card driver, but it has been greatly modified. This code assumes a IIe with 80 column card. For display, I had a lot of issues getting CR not to send a LF, but still return to beginning of the line. I also implemented a bare bones bell that sounds only if no characters are coming in.

I suppose a custom “fast” scroll routine might help with increasing the window size, but I haven’t had time to work on that. I think I left enough of the original Apple II peripheral card software mechanism in place, that it wouldn’t be too hard to move the code to PROMs that would reside on the serial card itself. For now, it assumes slot 2 for the serial card, and slot 3 for the 80 column card. Let me know if you have any luck with this.

I’m using a modified version of this software as the basis for my Apple II RTTY software, but the transmit side of that implementation needs some work. I want to create a way to type in some canned strings for transmitting CQ with a call sign and so forth.

2 Responses to “Improved Apple Teletype Emulation”

  1. Uncle Warthog says:

    Maybe you’ve answered this question in a previous post, but have you thought about using a different card, either a super serial card or one of the older communications interface cards (with the 6850 ACIA) and something to convert RS232 to current loop? I know there are converters available from Black Box and similar and they’re expensive but there are also some fairly good schematics out there for building your own as well.

  2. Mike says:

    yes, a UART based card, would make for a simple driver, assuming the clock can be set for 45.45 baud, but I’m having fun doing the bit banged version and would prefer avoiding having an external dongle/converter. Later on, I plan on porting this capability to the Apple 1 and the SCELBI/8008, so the more advanced card would require making a UART solution for those platforms.