Power Supplies

RS-232 Level Shifter with Isolation Schematic Circuit Diagram

Creating an RS-232 Interface with TTL Voltage Levels

This circuit serves as a bridge, converting TTL voltage levels on a UART to the appropriate levels for an RS-232 interface. Notably, it ensures galvanic isolation between the two sides of the circuit. While commercial ICs from manufacturers like Maxim offer similar functionality, they come with a considerable price tag. The circuit theoretically supports a maximum speed of 4800 baud. However, practical tests in the lab indicated a limitation to around 2800 baud. This limitation arises from the optocoupler’s switching time, specifically 15 µs (Ton) or 30 µs (Toff), as specified in its datasheet. Adjustments to the value of R4 might yield worthwhile results and warrant further experimentation.

RS-232 Level Shifter with Isolation Schematic Circuit Diagram

Galvanic Isolation and Power Distribution

The presence of the optocoupler inherently divides the circuit into two distinct and externally powered sections. The right-hand segment of the circuit draws power from the DTR and DSR signals on the RS-232 interface (pin 7 and pin 2 of K1). It’s crucial to note that these handshake signals must remain unused by the connected device, maintaining a permanent +12 V state. Since negative signaling voltages are generated on this side, a straightforward positive supply isn’t sufficient. To address this, the circuit utilizes a smart approach: tapping off the TXD signal on the RS-232 interface via diode D2. In its quiescent state, the connected device holds this signal at –12 V. To handle occasional signal highs during transmission, C1 smoothens the negative supply, ensuring stability.

Transmission Handling and Signal Inversion

When data is transmitted to the RS-232 interface, the TXD signal on pin 5 of K1 directly drives the diode in the optocoupler through series resistor R1. However, when pin 5 drops to –12 V, it’s essential to limit the reverse voltage across D1 to the permissible maximum of 6 V, as stated in the datasheet. In instances where transmission originates from the TTL level side. The receiver transistor in the optocoupler activates a buffer, constructed here using four NAND gates. Each gate operates as an inverter, effectively avoiding overall signal inversion. To enhance output drive, three of the gates are wired in parallel.

Power Supply and Logic Compatibility

IC1 relies on a ±12 V supply, necessitating the use of a CMOS device compatible with these voltages. While various logic functions besides NAND could serve the purpose, the chosen gates must facilitate wiring together to create a driver. Although individual CMOS gates like the TC4S81 are viable alternatives, these might be less common in an average experimenter’s toolkit compared to the readily available 4011 chip. The 5×2 header K1 can be seamlessly connected to a 9-way insulation displacement D-sub socket using a flat cable, following the specified wiring configuration.

RS-232 Level Shifter with Isolation Schematic


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