New monitors on old machines

I don't know if you do wiring the same way across the Pond, but over here the classic 2-way switch circut has a SPDT (changeover, form C) switch at each end, The fixed contacts are linked, that way there's a connection betwwen the 2 moving contacts if the switches are set to the same positions, and no connecion if theyr'e in opposite positons. To get a 3-way switch you conenct an ;intermediate switch' in the 2 wires that link the fixed conatacts of the 2 end switches. This switch is a 4-terminal device (obviously) and is electrically the same as a reversing swithc (that is, a DPDT switch with the fixed contacts of the 2 poles cross-coupled). Since the intermeidate switch effectively reverses the connections between the fixed contacts of the end switches, what you end up with is a circuit where : With the intermediate swithc in one state, theres a connection if the 2 outer switches are set the same way And with the intermediate swtich in the other state there's a connection if the 2 outer switches are set in opposite ways. A moments though will make you realise this is nothing other than a 3-input partity generator. And a bit more thought will show you that you can have as many intermediate switches as you want. So a 4 way circuit can be obtained by just adding one more intermediate switch. -tony