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CMOS to RS232 and RS232 to CMOS Converters Explained

rs232 cmos interface signal conversion voltage level

This document explains the concepts behind CMOS to RS232 and RS232 to CMOS converters.

CMOS to RS232 Converter

The following image shows a simple CMOS to RS232 converter. This type of converter translates CMOS signal levels to RS232 signal levels, enabling communication between devices using different signaling standards.

CMOS to RS232 converter

Figure 1: CMOS to RS232 converter

The converter leverages a MAX232 IC for the conversion between RS232 and TTL (Transistor-Transistor Logic) voltage levels, which is essential for compatibility.

CMOS Side Pin Descriptions

Table 1 details the pin assignments and signal designations for the CMOS side of the converter. These are typically connected to a DB9 Male connector.

CMOS Pin Descriptions
TXD- 3 (Output)
RXD- 2 (Input)
RTS- 7 (Output)
CTS- 8 (Input)
GND 5 (Signal Ground)

Table 1: CMOS Side Pins (DB9 Male)

RS232 Side Pin Descriptions

Table 2 outlines the pin assignments and signal designations for the RS232 side of the converter, often linked to a DB9 Female connector.

RS232 Pin Descriptions
RXD 3 (Input)
TXD 2 (Output)
CTS 7 (Input)
RTS 8 (output)
GND 5 (Signal Ground)

Table 2: RS232 Side Pins (DB9 Female)

Construction Notes

To build a CMOS to RS232 converter, make the connections as described in the tables above, ensuring a one-to-one mapping. Critically, you must account for the voltage differences between RS232 and CMOS logic levels for “HIGH” (logic 1) and “LOW” (logic 0) signals, as summarized in Table 3.

RS232 to CMOS Converter

The RS232 to CMOS converter performs the reverse function, transforming RS232 signal levels into CMOS signal levels. The same cable configuration described above can be utilized with appropriate mating connectors.

Logic Level Comparison

Table 3 provides a crucial comparison of the voltage levels for logic 1 and logic 0 in both RS232 and CMOS signaling.

Logic TypeRS232CMOS
Logic 1>+5V (at output), >+2.4V(at input)>2.4V (at output), >2V (at input)
Logic 0<-5V (at output) <0.2V (at input)<0.55V(at output), <0.8V (at input)

Table 3: Voltage Level Comparison

Applications of CMOS to RS232 Converters

CMOS to RS232 converters are valuable in a variety of applications:

  • Providing RS232 to CMOS and vice-versa adaptation.
  • Typically handling four signal conversions.
  • Supporting baud rates up to 115.2 Kbps.
  • Often operating without an external power supply, deriving power from the connected device (e.g., laptop, PC).