Serial keyboard interface

7-22 — elektor july/august 1982 With a bit of luck it is sometimes possible to purchase a high quality keyboard without having to pay too much for it. Most of these keyboards have a parallel output that supplies an ASCII or Baudot code. Trying to connect it to a personal computer will cause some problems because most computers are equipped with a serial RS 232 interface. The circuit described in this article will provide the solution to this problem; It converts a parallel ASCII or Baudot code into a serial signal. The signal conversion is performed by a UART of which only the transmitter is used. The Baud rate is produced by a clock generator which is constructed using the well-known 555 timer. The clock frequency must be 16 times the Baud rate. The serial data signal is situated at pin 25 of the UART and is boosted to the RS 232 level by way of transistor Ti. The lenght of the serial "word" can be set with the aid of the logic levels at pins 37 and 38. The logic level at pin 3...