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Signal chain basics: Using RS-232-to-RS-485 converters for industrial long-haul comm

Posted: 27 Jan 2012 ?? ?Print Version ?Bookmark and Share

Keywords:RS-232-to-RS-485 converters? RS-485? RS-232?

Industrial data links requiring the transmission of RS-232 data over long distance, or between multiple RS-232 applications, often use RS-232-to-RS-485 converters. Despite the wide and high signal swing of up to 13V, RS-232 is an unbalanced (single-ended) interface and, as such, is such highly susceptible to noise. Its bus maximum length is therefore limited to about 20m (60 ft). While allowing for full-duplex data transmission, that is transmitting and receiving data at the same time via separate signal conductors, RS-232 does not support the connection of multiple nodes on the same bus.

In strong contrast, RS-485 is a balanced interface using differential signaling, which makes it highly immune to common-mode noise. Therefore, extending an RS-232 data link over long distance and enabling the connections of multiple bus nodes requires the conversion to RS-485 signals via interface converters (figure 1).

Figure 1: Converting short distance, point-to-point data links into a long-distance, multipoint network. (Click on image to enlarge.)

Figure 2 shows the schematic of a low-power, isolated-converter design. Here, the RS-232 serial port of a personal computer (PC) for example, connects to the subminiature-D9 female connector on the left.

The PC serial port contains a RS-232 driver, and receiver chip which converts its internal 5V logic signals to the higher 8V to 13V levels at the connector. These high-voltage bus signals are converted back to standard logic levels via another RS-232 chip, in order to communicate with the RS-485 transceiver.

In transmit direction, the 485-transceiver converts the logic signals from the RS-232 receiver output into differential bus signals. In receive direction, it converts the differential bus signals into single-ended, low-voltage signals entering the RS-232 driver input.

The RS-485 transceiver includes a capacitive isolation barrier which galvanically isolates the bus side from the logic-control side, which eliminates ground currents between the bus nodes.

On the bus side, the converter design provides several components to ensure reliable data transmission. Jumpers J1 and J2 activate a failsafe biasing network during bus idling. Via jumper J3, a 120? termination resistor can be implemented, if this converter is located at a bus end.

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