and CP/M was chosen for two reasons. Firstly the University of York 
Electronics Department were prepared to offer a support service for 
such à machine, and secondly it was known that an 'off the shelf' 
interface was available which would work with this specification of 
machine. There are a bewildering variety of microcomputers available, 
even within this specification, all with their own claims to have some 
unique feature. The North Star seemed to offer good value for money, 
plus a local agent for servicing. 
Interface 
The interface used between the encoders and the microcomputer was an 
"off the shelf' item manufactured by Soton Micro Systems. Most of the 
electronic components for this interface are on one card which slots into 
the North Star S100 bus. A separate power supply unit is needed to 
provide current for the encoders. The coordinate display and menu are 
shown only on the VDU. There are no separate counter displays on nixie 
tubes or LED etc, The software supplied as part of the interface 
'package'! provides a range of options for recording in point, time or 
distance mode, and facilities to insert headers before strings or 
coordinates, etc. A certain amount of alteration to the software had to 
be carried out by the manufacturers to make it operable with the North 
Star/Hazeltine combination. Coordinates can be stored onto either of the 
disks, or output to another device if wanted. 
OUTPUT 
The function of the output system is to electrically drive the Thompson- 
Watts Coordinatograph table through stepper motors. This output is 
perhaps the more interesting part of the whole system, as it shows how 
for a very modest cost, an existing coordinatograph table can be driven 
electrically. Fig 5 shows a general view of the coordinatograph table, 
with the stepper motor unit attached. 
Fitting the Stepper Motors 
  
The 'X' and 'Y' shafts of the coordinatograph conveniently projected about 
15 mm from the body of the table (Fig 4). A drive system was designed 
using a flexible toothed nylon belt, working with a two to one reduction 
(Fig 6). This allowed the motors to be set back so (Fig 7) that they did 
not project unnecessarily, and also allowed much lower tolerance in the 
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