This design is for a simple half-duplex optically isolated interface that converts a 20 mA current loop (connected to J2) into an RS232 signal (on J1) which can be monitored by, say, a laptop PC. In the author’s case, the system operates at 1200 baud. The signal to be monitored should be a fully digital on-off communications signal, rather than the usual 4/20 mA (industrial) analogue transmission standard. The overall action of the interface is of double inversion.
The current in the comms signal is normally present when no data is being sent, and the current is switched off to represent data. Consequently, the transistor in the optoisolators normally switched on, giving a low at the input to IC1c. This is inverted to give a high (+12 V) on the RS232 input, which is the default condition for no data. The interface itself is powered by the serial (RS232) port used to monitor the comms signal.
This is achieved by stealing power from unused RS232 signal lines. The standard RS232 connector is a 9-way male ‘D’ type whose connections are shown in the table. The positive and negative supply rails for IC1 are set up by rectifying the unused RS232 potentials via diodes D1 through D6, with C1 and C2 acting as reservoir capacitors. Opto-isolator devices normally switch on reasonably fast but are relatively slow to switch off. Resistor R2 speeds up turn-off time.
Diode D7 has been included to protect the optoisolator against excessive reverse voltages — these may occur when the interface is accidentally wired back to front. If voltage drive is used instead of 0/20 mA pure current drive, a current limiting resistor is required at the opto-isolator input. This resistor will typically be between 330 Ω and 1 kΩ, and the LED current should always be kept well below 50 mA to prevent damage to the opto-isolator. The circuit may be modified for compatibility with 4/20 mA industrial current-loop systems by carefully matching the value of R2 to the opto-isolator used. In general, the lower the value, the less sensitive the interface will become.
Almost any opto-isolator device may be used provided its transfer is close to 100% (or ‘1’ — check datasheets). Good results were obtained with, among others, the Siemens CNY17-2. This device boasting a breakdown voltage specification of 5,300 V, it is Class-2 compliant provided the distance between the pins is greater than 6 mm. This however will require some bending. For Class-1 safety requirements, the normal pin distance governed by an 8-way DIL socket is adequate.
