| 摘要 |
A universal I/O circuit is provided to interface a control computer system with remote sensor transmitters which produce outputs representing one or more process variables and with remote controlled devices which control process variables. The circuit uses a pair of terminals in series combination with an electrically-controllable variable resistance device and a load means, across which series combination a supply voltage is applied. A microcomputer controls the resistance of the variable resistance device, as well as of associated circuitry, to operate them appropriately to receive current inputs from the remote transmitters or to supply current or voltage outputs to the remote controlled device; in any of these cases, the remote controlled device or remote transmitter is connected directly to the two terminals. The variable-resistance device is preferably a transistor, and the load means a resistor. A third terminal may also be provided to receive voltage inputs from a remote device, which voltage input is applied between the third terminal and the terminal which is connected to the load means; the microcomputer again controls the I/O circuit to provide appropriate operation. Either continuous analog or discrete on-off signals may be accommodated, and two such I/O circuits may be jumpered together at their I/O terminals to accommodate bipolar signals when, as is preferred, the individual I/O circuits are otherwise galvanically isolated from each other.
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