The circuit described here implements remote/local transfer of control for distributed-control-system equipment. I have used this circuit as a part of the electronic circuitry for the remote/local control of a laser system. The laser can be controlled from a remote computer system located away from the laser system, or from a local control unit that’s actually mounted on the system.
The circuit should be used in distributed control systems that are controlled both locally and remotely. Such systems should normally run under remote control, and in this case, local control should be disabled. Local control is possible only with the permission of the remote controller. Also, once the equipment is put under local control, the remote system should not take over the control on its own unless the local control is released by the operator.
This circuit should be mounted in the equipment’s local-control panel. In-system remote/local enable signals are connected to chip select pins of tristate buffers, which allow either remote or local commands to control the equipment.
U1/2 and U1/3 form a basic flip-flop, triggered by the logic function generated by U1/1. When the local enable control signal goes high, transistor Q1 is on and Q2 is off. Capacitor C1 charges to logic high level, and input B of U1/1 goes high. The input A of U1/1 and of U1/3 changes to a logic high when the local mode switch is closed. Then the output of U1/1 goes low and the output of U1/2 goes high, causing the output of U1/3 to go low. U1/4 inverts this signal and enables local mode. Now even if the local enable control signal goes low when S1 is closed (this happens if the remote computer tries to take over the control once the local mode is on), the local mode of the equipment control isn’t disturbed. That’s because the output of U1/2 and U1/3 remains unchanged to high and low state, respectively. Thus, once local mode is established, control can’t be reclaimed by the remote computer unless it’s released from local-control mode manually.
When the remote control is on (i.e. S1 is off and the local-enable-control signal is low), the A and B inputs of U1/1 are low, making the output high. Because the A input of U1/3 is low, its output is high and remote mode is on. In this case, if switch S1 is closed, the A input of both U1/1 and U1/3 goes high. However, the outputs of U1/1, U1/2, and U1/3 don’t change. Consequently, once the equipment is in remote mode, it can’t be put in local mode without permission of the remote controller. Permission to go into local mode is given by making the local-enable signal high. A panel indicator (LED1) may be used to indicate the status of the local-enable control signal.