Gray Code and Circuit Diagrams

Gray Code

This code was originally devised to optically encode shafts. The key to it is that only one data line ever changes state at a time. This helped eliminate false pulses occurring when two leading edges on an optical feedback system could not be synchronised perfectly.

It turns out that this is also an ideal way to drive small stepping motors. The type of motor that is found in the head transports of floppy disc drives are a cheap and practical source of these motors for our experiments.

The motors are small and easy to power for simple experiments. Even using a the simplest chip of all, a TTL-Logic 7404 hex inverter is capable of turning a motor over. The power and speed of the motor due to the low currents supplied by the 7404 chip limit the motor considerably, but never the less do demonstrate the principle of 'Gray Code' and the precision of a stepping motor over a free running motor.

The Printer Port

The printer port on a computer has become a defacto general input output port on many computers. Some have scanners, floppy disk drives, SCSI drives, ZIP drives and even printers in some cases! This is all very well, but it has meant that specialist I/O ports on the computers with any genuine PIA style interface has been sadly lacking. However the printer port can also be pressed into service by us as well. Quite a deal can be achieved even with this limited resource.

In fact the limitations of the port and the fantastic power of the computer to produce a dazzling array of software commands to overcome these limitations is what these series of web-pages is all about.

The Gray Code

The above diagram demonstrates the sequence of pulses that must be output on the printer port pins to produce a rotation on the motor. The direction is chosen just for the example and the opposing direction is given below.

The El-Cheapo Starting point

People who understand these diagrams will recognise that these motors are not energy misers. While they are connected there is always current flowing in both coils. The direction of the current thought the coils is simply alternated according to the Gray code and the permanent magnet armature is forced to rotate in the center. There are no brushes or commutator. This setup above will turn the average disk drive head motor OK but has little power due to the output limitations of a hex-inverter. Cheap though!

A Line Driver Chip

This chip is a line driver that was used in the 'Econet Interface' of the BBC32k Computer made by Acorn. Anyone who has run a network of any dimensions of these machines will probably still have a few spare. They produce a compact and fairly powerful driver for the bipolar motor. They will get quite hot at at times but don't seem to be damaged by it all. Any other line driver that has similar characteristics could be pressed into service. If you know of one please let me know!