The obvious way to keep this availability was to use exactly the same connectors as on the old system so that either system could be plugged into the commonly used equipment relatively easily. There were two places where this could not be done.
a) The output of the stepper driver unit was via a 30 way connector. This connector was not available in any catalogue from our usual suppliers so instead I cut the cable near the interface box end and put a plug and an in-line socket in it. The plug is a permanent fixture, used to plug into the new system, while the socket was only temporarily on the old system until it was officially declared obsolete and broken up.
b) The output from the old BBC computer to the relay driving the pneumatic solenoid was via a screened cable and a BNC socket, the screening being ground and the central pole carrying a 5V signal to energise the relay. The new PC system was going to use an open collector output which meant that one side of the relay needed to be connected permanently to V+ and the signal would energise it by pulling the other end of the relay down to ground. This would cause problems with the new system because ground on the BNC socket was connected to the earthed case of the relay box. The box was earthed for safety reasons as 110 V mains was being switched by the relay. This connector was therefore changed over to a 6 way type so that neither wire would need to be connected to the case.
With these modifications, work on the PC project was able to proceed whilst retaining the availability of the existing BBC computer run equipment.