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Power Supply
In the majority of cases the circuit can be powered by the printer, in which case SK4 must be linked to SK5, and IC8 omitted. If the printer does not have a 5V output this link should be omitted, and the circuit powered by a 5V supply regulated by IC8. The power input to SK6 should be between 7.5V and 10V DC at about 150mA max. This does not need to be regulated, so a simple mains adaptor is adequate. The DC input is not protected against reverse polarity.

Note that SK3 is a 40 way IDC connector on the PCB. This is connected to a 36 way centronics plug with IDC ribbon cable. The inside numbers on the circuit diagram relate to the 40 way IDC connector, and the outside numbers to the 36 way centronics plug.

Construction
All the components except the LED's mount on a single sided PCB. The component overlay is shown in figure 2 and track layout in figure 3. The board is fairly tightly packed with many close tracks, so take due care when constructing it. It may be worth checking your work with a magnifying glass when you've finished.

Before starting, drill out the four mounting holes to 3mm (1/8"). There are a large number of wire links, and these should be fitted first since many of them pass underneath other components. Thin tinned copper wire (about 28 SWG) is ideal. With thicker wire it may be more difficult to avoid short circuits unless it is sleeved.

The remaining components may now be fitted, in any convenient order. Sockets may be used for the IC's if desired, although LS TTL is unlikely to be damaged by soldering. On the prototype, the LED's (D5 and D6) were mounted on the case. If you wish to do the same, fit terminal pins in their positions on the board.

SK1 and SK2 should be fitted facing towards the edge of the PCB. When fitting SK3, be sure to get the 'V' indicator which marks pin 1 in the position shown. If in doubt, the large polarising gap goes towards the centre of the PCB.

You now need to establish whether your printer has a 5V output on pin 18 of the centronics connector. If you have the manual, check the section towards the back that gives the interface connections. Otherwise you will have to get your test meter out!

If you have a 5V output, link pads SK4 to SK5, and omit C8, IC8 and SK6. If you don't have a 5V output fit the above components and omit the link. Terminal pins should be fitted for SK6. IC8 will not need a heatsink if the unregulated supply is below about 10V, otherwise you will need to fit one and you may have to find a rigid method of mounting the pair.

Ribbon Cable
The PCB is connected to the printer with a piece of 40 way ribbon cable, no more than about 1 metre long. Fit the 40 way IDC socket to one end of the cable. The red stripe on the cable indicates pin 1, and should go to the 'V' mark on the connector. Position the cable so it emerges on the side of the connector with the polarising bump. The strain relief clip should now be fitted which reverses the cable direction.

Now separate wires 37 through 40 - these are the four farthest from the red wire. Cut these four off close to the 40 way IDC socket. Now fit the 36 way Centronics plug to the other end of the cable, again with the red wire towards pin 1.

Ribbon cable is best cut with a sharp pair of scissors. Try to cut it square. You will probably need to trim both ends to avoid stray strands causing short circuits. The connectors can be carefully pressed together using a WorkMate or vice. If using a metal vice, use couple of pieces of wood to protect the connector. Make sure the connector is squeezed evenly and the cable is square.