Derek Walker provides the know-how on a new hardware device to allow
two printers to be connected to your Electron
This project, a printer switch which will direct the output of your computer to one of two printers, is best suited for the computer/electronics hobbyist. I have simplified the construction by designing a printed circuit board (PCB). The layout will appear in the second part of this article (in EUG #6) but if you're impatient the circuit can be built on Vero board.
Operation
Please refer to the circuit diagram when reading the following text.
When the computer is asked (commanded) to print data, eg from View, the data appears on pins 2 to 9 of SK3. U1 and U2 (74HCT245) are used to block (or let pass) the data to one printer. S1 is used to enable U1 while it disables U2 and vice versa.
Two other signals, the strobe (STB) and acknowledge (ACK), used for handshaking are buffered and selected by U3 and S1. If S1 is set for printer one then the STB and ACK will be routed to/from SK1 or if S1 is set to printer two the signals will be routed to/from SK2.
The entire circuit consumes very little current, therefore it is possible to power the integrated circuits from the data bus and STB and ACK lines. C1 is used to store and smooth the 5V.
| Part No. @ Maplin Electronics | Order No. | Qty | Price |
| 26 Pin Header (SK1,2) | JB95D | 2 | £0.86 |
| Centronics socket (SK3) | FV88V | 1 | £1.98 |
| 74HCT245 (U1,2) | UB68Y | 2 | £0.65 |
| 74HCT244 (U3) | UB66W | 1 | £0.56 |
| 4K7 0.6W Resistor (R3-R6) | M4K7 | 4 | £0.03 |
| 2K2 0.6W Resistor (R1,2) | M2K2 | 2 | £0.03 |
| 10uF 16V Tant. (C1) | WW68Y | 1 | £0.28 |
| Switch 2p 6w (S1) | FF73Q | 1 | £0.98 |
| Knob | FD68Y | 1 | £0.38 |
| Wire | BL85G1 | 1 reel | £0.42 |
| Box - Maplin has several suitable cases starting at... | £2.65 | ||
| TOTAL (Inc. VAT) | £10.45 |
Notes on the Printer Switch Circuit Diagram
- Even pin numbers, 2 through to 24 on printer sockets should be connected to 0V
- Connect pin 10 of U1, U2 & U3 to 0V
- Connect pin 20 of U1, U2 & U3 to 5V
Connection
The centronics printer cable which connects the Plus 1 to the printer is unplugged from the printer and plugged into SK3, the centronics connector on the PCB. The two printers are then connected via new centronics cables to SK1 and SK2.
In part two - construction and testing details.
Derek Walker, EUG #5