SIGNAL CHAIN BASICS #57: RS-232-to-RS-485 converters used in industrial long-haul communication

(Editor's note : Signal Chain Basics is an ongoing (and popular) series; you can click here for a complete, linked list of installments #1 through #55 of the series, and here for all installments.)

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 (see Figure 1 ).

Figure 1. Converting short distance, point-to-point data links into

a long-distance, multi-point network.

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.


Figure 2. Isolated RS-232 to RS-485 converter with auto-direction control

(click here to see enlarged image).

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.

A transient suppressor protects the bus transceiver from dangerous transient over-voltages by clamping them to ground potential. In order to divert transient currents to Earth potential, a high-voltage capacitor is required to provide AC-coupling between the floating bus ground and protective Earth (PE). Typically a short single conductor (18 AWG) is used to make the connection to a PE terminal or chassis ground.

Isolating the signal path also requires isolating the power supply. Here, the bus supply (3.3V to 10V) is regulated via a low-dropout voltage regulator (LDO). Then it is applied to the transceiver bus supply (Vcc2) and to an isolated DC/DC converter. This converter consists of a transformer driver, an isolation transformer, and a second LDO which supplies the circuits on the logic side.

Older converter designs sometimes use a request-to-send signal (RTS) to switch the RS-485 transceiver from receive into transmit mode. In PC applications, however, the RTS generating interface software runs under Windows® and not in real-time. Thus, if Windows decides to use its processing time for another application, such as a screen saver or antivirus software, RTS might not change the transceiver back into receive in time, and data sent by another bus node might be lost.

The converter design in Figure 2 eliminates this possibility by implementing an auto-direction function. The auto-direction detection is accomplished with a monostable flip-flop, whose output is triggered high by the start bit from the RS-232-receiver output. By default, the RS-485 transceiver is in receive mode. When the monostable output goes high, it switches the transceiver into transmit mode.

The time constant of the monostable output is defined by an RC network with C=220 nF, and R=10 k?, for a 2 ms high-time at a data rate of 9600 bps, and R=100 k? for 20 ms at 1200 bps. When the high-time has passed, the monostable output returns to low, thus switching the transceiver back into to receive mode. While the auto-direction function is data rate dependent, it is a reliable method to prevent data loss.

In a future article, we will discuss multi-protocol circuits which allow the transmission of either RS-232 or RS-485 signals across the same interface lines. Meanwhile, please join us next month when we will address how to use SPICE to optimize the right-leg drive amplifier for reducing common mode noise in an ECG front end.

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About the author

Thomas Kugelstadt is Applications Manager at Texas Instruments where he is responsible for defining new, high-performance analog products and developing complete system solutions that detect and condition low-level analog signals in industrial systems. During his 22 years with TI, he has been assigned to various international application positions in Europe, Asia and the U.S. Thomas is a Graduate Engineer from the Frankfurt University of Applied Science. Thomas can be reached at .

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