RS232, RS422 and RS485 are simple asynchronous serial protocols used in a wide range of commercial, industrial, and military applications. All three use the same basic logical protocol, but vary in their physical implementation, i.e., voltage levels and signaling. Since both protocols are commonly used for the same or similar applications, it is often desirable or necessary to provide signal conversion between them. In many cases the conversion is required to interface an RS422/RS485 device to the RS232 port on a PC.
RS232 is the simplest interface, which consists of 3 wires: TX, RX, and GND. For the purposes of this discussion we can ignore the optional handshaking signals; they are rarely used and don’t apply to RS232 to RS422/RS485 conversion. RS232 supports full duplex communication between two devices over the TX (transmit) and RXD (receive) lines as shown in the figure below. Signaling voltage levels for RS232 are +3V to +15V for logic “0” or “space” and -3V to
-15V for logic “1” or “mark”. In practice nearly all modern devices us +5V for logic “0” and -5V for logic “1”.
RS422/RS485 use differential signaling and cable termination to increase interface speed and noise immunity. RS485 also supports half duplex and multi-drop configurations for networking multiple devices, but these configurations are beyond the scope of this discussion. For RS232 to RS485 conversion purposes we will only consider “point to point” RS422/RS485 configurations with two devices; in this case RS422 and RS485 can be considered electrically identical.
RS422/RS485 differential signaling uses a pair of wires to transmit voltages that represent logic “0” and logic “1”. The transmitter applies voltages to the TX+ and TX- signals which the receiver interprets by subtracting the RX- signal from the RX+ signal. The differential voltages can range between +/- 200mv and +/- 6V, with positive voltage for logic “1” and a negative voltage for logic “0”. In practice the differential voltages are almost always +/- 5V. For +5V or logic “1” TX+ is +5V and TX- is 0V or GND and for -5V or logic “0” TX+ is 0V or GND and TX- is +5V. \
RS232 to RS422/RS485 conversion
RS232 to RS422/RS485 conversion is normally accomplished using commercially available adapters that use RS232 and RS485 transceiver IC’s to provide the required voltage signaling translations. A functional block diagram for a typical RS232 to RS422/485 conversion adapter is shown below. The main drawback to these adapters that they require an external power supply, usually 5V a wall transformer, to provide power to the transceiver IC’s.
For many RS232 to RS22/485 applications, particularly those that operate with cable lengths and baud rates that are within the RS232 specifications, it is possible to make cable connections that provide the conversion without additional electronics. The required connections are the subject of one of our upcoming blog entries.
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