Signal Chain Basics 57 RS-232 to RS-485 Converters for Industrial Long-Distance Communications

Some industrial data links that require RS-232 data transmission over long distances or between multiple RS-232 applications usually use RS-232 to RS-485 converters. Despite the high signal swing of up to ±13V, RS-232 is still an unbalanced or single-ended interface and is inherently very susceptible to noise. Its maximum bus length is limited to around 20 meters (60 feet). While allowing full-duplex data transmission (sending and receiving data simultaneously over some separate signal wires), RS-232 does not support connecting multiple nodes on the same bus.

In stark contrast, RS-485 is a balanced interface that uses differential signaling, giving it high immunity to common mode noise.Therefore, extending the transmission distance of the RS-232 data link and realizing the connection of multiple bus nodes requires that it be converted into an RS-485 signal through an interface converter (seepicture1).

Signal Chain Basics 57 RS-232 to RS-485 Converters for Industrial Long-Distance Communications

picture1 Conversion of Short-Range, Point-to-Point Data Links to Long-Range, Multipoint Networks

picture2A schematic of a low-power, isolated converter design is shown. Here, the RS-232 serial port of a personal computer (PC) is connected to the SUB-D9 connector on the left.

picture2 Isolated using auto-direction controlRS-232arriveRS-485converter.

The personal computer serial port contains an RS-232 driver and receiver chip that translates its internal 5V logic signal to a higher ±8V to ±13V level at the interface. These high voltage bus signals are then converted back to standard logic levels by another RS-232 chip to communicate with the RS-485 transceiver.

In the transmit direction, the 485 transceiver converts the logic signal from the RS-232 receiver output to a differential bus signal. In the receive direction, it converts the differential bus signal into a single-ended, low-voltage signal that goes into the input of the RS-232 driver.

The RS-485 transceiver includes a capacitive isolation layer that provides galvanic isolation between the bus side and the logic control side, thereby eliminating ground currents between bus nodes.

On the bus side, the converter design has several components that ensure reliable data transmission. Jumpers J1 and J2 activate the failsafe biasing network during bus no load. If this converter is installed on the bus side, a 120 ohm termination resistor can be implemented via jumper J3.

A transient suppressor protects the transceiver from dangerous overvoltage transients by clamping to ground potential. To shunt transient currents to ground potential, a high voltage capacitor is required to provide AC coupling between floating bus ground and protective ground (PE). Typically, we use a short single-conductor wire (18 AWG) to make the connection to the PE terminal or chassis ground.

Signal path isolation also requires power isolation. Here, we regulate the bus power (3.3V to 10V) through a low dropout voltage regulator (LDO). Then, use it for transceiver bus power (Vcc2) and an isolated DC/DC converter. This converter consists of a transformer driver, an isolation transformer, and a secondary LDO that powers the logic side circuits.

Older converter designs sometimes use a request to send signal (RTS) to switch the RS-485 transceiver from receive mode to transmit mode. However, in some computer applications, the RTS generation interface software runs under Windows® and is not real-time. Therefore, if Windows decides to dedicate its processing time to another application, screen saver, or antivirus, the RTS may not be able to switch the transceiver back to receive mode in real-time, so the data sent by the other bus node may be lost.

picture2The converter design shown eliminates this possibility by implementing an automatic direction selection function. This automatic direction selection detection is realized by a monostable flip-flop. The output of the flip-flop is triggered high by the 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. At a data rate of 9600 bps, C = 220 nF and R = 10 kOhm at a 2ms high time; at a data rate of 1200 bps, with a 20ms high time, R = 100 kOhm. When the high time expires, the monostable output goes back low again, switching the transceiver back to receive mode. Although auto-direction is dependent on the data rate, it is still a reliable method of preventing data loss.

Next time, we’ll discuss multiprotocol circuits. This circuit allows RS-232 or RS-485 data transmission between interfaces of the same type. Also, stay tuned next month as we cover how to use SPICE to optimize the right leg driver amplifier to reduce common-mode noise on the front end of an electrocardiogram (ECG).

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