Scor­pio News


April–June 1987 – Volume 1. Issue 2.

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using software, we reconfigure the CTC to use the divider mode and system clock for 1200/75 baud, and the counter mode and external clock for the faster baudrate of 9600.

The configuration tables for the CTC are:

To switch between 1200/75 and 75/1200 takes a little more work – I had to design a patch for my MODEM, and for CP/M Plus, but it is possible. I hope that this long rambling article will persuade others to try using an unused channel of their CTC – it is a most useful chip. I know of one system where an unused CTC bas employed to provide dooropen interrupts for three drives, and the fourth channel was used to provide a real time clock interrupt to update the on screen date and time every five seconds.

In the next article, I’ll write about the SIO and its programming.


  1. 80-BUS News Vol 2, No 1, The Interrupt Structure of the Z80.
  2. 80-BUS News Vol 1, No 3, Teach Yourself Z80 – Part 7
  3. 80-BUS News Vol 2, No 3, Serial Interface Problems made Easy
  4. Zilog Counter Timer Circuit Technical Manual
  5. Z80 Assembly Language Subroutines by Leventhal and Savile,
    Osborne/​McGraw-Hill, p385

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