Cheap Printout 1

by Mark Horsman

Having the usual need for cheap hardcopy, I thought myself a Creed 75R for about £20.

“Great.”, I thought, “Now how do I drive the thing?”

Personal Computer World, May 80 and Practical Computing, July 80 both had articles using one bit of a parallel port to output the 5 bit Baudot code, shifting one bit out at a time and generating the start and stop bits. This meant various timing loops that needed trimming, seemingly depending on the weather, not the most elegant solution, so I looked at the UART. In the Nascom 2 this is configured for 8 bit use only but is capable of 5, 6, 7 and 8 bit use depending on the voltages on pins 37 and 38. Surgery is called for. Cut he track between pins 39 and 38, cut the track between pins 37 and 35, join pins 39 and 35 with a short length of fine wire and add a 1K resistor between pin 1 and pins 37 and 38. This leaves us still in 8 bit mode. Now add a switch between pin 38 and ground such that when open we have 8 bits but when closed we have 5 bit mode. Next we need a circa 16 x 50Hz clock to drive the UART. This is generated with a 7555 (CMOS 555 timer) using a multiturn pot to trim the frequency. This is wired to PL2 pin 4 or TP4. The output is taken from PL2 pin 12 (20mA Loop Out).

Finally, the software. This uses a lookup table, a flag to determine whether letters or figures last sent and a routine to convert such as > to .GT., etc.

P.S. If you live in a terraced house, move – the noise is horrendous.