You should have no trouble finding one or more of the thousands of examples of bit-banging a serial interface that have been published over the years. A GOOGLE search for "software UART" produces lots of hits. It's not rocket science. Keep in mind that with asynchronous communications, you need to remain within one bit-time over the course of, at most, 11 bits, of which the first and last are start and stop bits, respectively. If you stick with 7-bit ascii, you have more margin. Each time you start, you start anew, so there's no cumulative timing error over more than one character. If you don't mind the code size, you can time this VERY well regardless of the main oscillator frequency, since you can make minor adjustments between bits. It's normal, BTW, to sample at the middle of a bit window. (someone had to point that out to me once ... but only once ... ) The Z80 computes parity in the PSW, though there's some sort of gotcha with that, if my feeble memory serves. good lluck. Dick ----- Original Message ----- From: "Stan Barr" <stanb(a)dial.pipex.com> To: <classiccmp(a)classiccmp.org> Sent: Sunday, March 24, 2002 2:43 AM Subject: Re: ZX81 software serial port (was Re: ZX81 I/O ports)

The SWTPC 6800 computer system used a bit banger serial port based on the Motorola MIKBUG ROM monitor. It was limited to 110 or 300 baud. The I/O card (SWTPC MP-C) use a 6820 parallel interface. It was great when SWTPC released a new ROM that allowed for a real serial chip (6850) and you could get your TV Typewriter going at 1200 baud. I later used my MP-C card to interface to an IBM Selectric terminal at 134.5 baud. I had to convert the ASCII to IBM code in software anyway. Motorola MIKBUG http://www.swtpc.com/mholley/MP_A/MP_A_Index.htm SWTPC 6800 System http://www.swtpc.com/mholley/swtpc_6800.htm A 1978 picture of me at my Trendata 1000 Selectric terminal (IBM 2741 compatible). http://www.swtpc.com/mholley/MySystem/MySystemPhoto.htm Michael Holley www.swtpc.com ----- Original Message ----- From: "Richard Erlacher" <edick(a)idcomm.com> To: <classiccmp(a)classiccmp.org> Sent: Sunday, March 24, 2002 7:00 AM Subject: software UART