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Opto-Isolated RS232 Interface Schematic Circuit Diagram

Creating a Basic Half-Duplex Optically Isolated Interface

This schematic outlines a straightforward half-duplex optically isolated interface designed to transform a 20 mA current loop (connected to J2) into an RS232 signal (available at J1). This RS232 signal can then be monitored, for instance, by a laptop PC. In the author’s scenario, the system operates at 1200 baud. It’s important to note that the signal to be monitored is a purely digital on-off communications signal, deviating from the typical 4/20 mA industrial analog transmission standard. The fundamental operation of this interface involves a double inversion process.

Opto-Isolated RS232 Interface Schematic Circuit Diagram

Comms Signal Handling and Default State

In the communication signal, the current is typically present when no data transmission occurs, and it is switched off to signify data transmission. Consequently, the transistors in the optoisolators remain switched on under normal circumstances, resulting in a low input at IC1c. This low input is inverted to produce a high voltage (+12 V) on the RS232 input, representing the default condition when no data is being transmitted. Notably, the interface draws power from the serial (RS232) port utilized for monitoring the communication signal.

Power Supply Configuration and Opto-Isolator Performance

To power the interface, power is drawn from the unused RS232 signal lines. The standard 9-way male ‘D’ type RS232 connector is employed, with its connections detailed in the table. The positive and negative supply rails for IC1 are established by rectifying the unused RS232 potentials through diodes D1 to D6, while C1 and C2 function as reservoir capacitors. Opto-isolator devices typically switch on relatively fast but exhibit slower turn-off times. Resistor R2 is integrated to enhance the turn-off speed of the optoisolators.

Reverse Voltage Protection and Voltage Drive Consideration

Diode D7 has been incorporated to safeguard the optoisolator from excessive reverse voltages. Which might occur if the interface is mistakenly wired in reverse. When using a voltage drive instead of pure 0/20 mA current drive, it becomes essential to include a current limiting resistor at the opto-isolator input. Typically ranging from 330 Ω to 1 kΩ, this resistor ensures the LED current remains well below 50 mA, preventing any damage to the opto-isolator.

Adaptation for Industrial Current-Loop Systems and Opto-Isolator Selection

To make the circuit compatible with 4/20 mA industrial current-loop systems, careful adjustment of R2 to match the specific opto-isolator used is necessary. In general, a lower value for R2 results in reduced sensitivity for the interface. Virtually any opto-isolator device can be utilized, provided its transfer rate is close to 100% (or ‘1’ as indicated in the datasheets). Optimal outcomes were achieved with devices such as the Siemens CNY17-2, which boasts a breakdown voltage specification of 5,300 V. To maintain Class-2 compliance, it is important to ensure the distance between the pins exceeds 6 mm, even though this might require some bending. For Class-1 safety requirements, the standard pin distance governed by an 8-way DIL socket is sufficient.


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