When networking multiple RS485 pressure transmitters, improper termination resistor or device address settings may cause communication problems. Properly configuring the wiring, termination matching, address allocation, and field troubleshooting is essential for stable operation of multiple devices on the same communication line.

1. Determine the Bus Topology and Communication Conditions

RS485 is an electrical interface standard. Multiple pressure transmitters can be connected to the same differential bus and polled in sequence by a master device such as a PLC, industrial computer, or data acquisition unit. Whether the devices can actually communicate also depends on their communication protocols and parameters; sharing the same interface alone does not guarantee that they can be networked directly.

A daisy-chain bus topology is recommended for field installations, with each device connected in sequence to the trunk line. Avoid long branches running in multiple directions from a central point wherever possible. Keep each transmitter’s drop line as short as possible to reduce signal reflections. The allowable drop-line length should also be determined based on the baud rate, cable, and device documentation.

Before networking, confirm that the master and transmitters use the same protocol, baud rate, data bits, parity, and stop bits. For example, some devices use Modbus RTU, but register addresses, data types, and pressure units may still vary by manufacturer or model.

What Should You Check When Wiring?

Use twisted-pair cable that meets the requirements of the installation, and connect it according to the terminal markings on the devices. A and B may be labeled with opposite polarities by different manufacturers. During commissioning, check the polarity definitions in the manuals rather than relying on the letters alone.

Keep communication cables as far as possible from high-interference lines, such as variable-frequency drive output cables and motor power cables. The grounding method for the cable shield should be determined according to the system design and site standards. Also consider the common-mode voltage between devices; for long distances or significant ground-potential differences, an isolated RS485 interface may be considered.

The transmitter power supply must meet the requirements on its nameplate and in its manual. The communication cable generally cannot replace the device power cable. Before wiring, disconnect the power and check the supply polarity, protective grounding, and terminal functions.

2. Where Should Termination Resistors Be Installed?

Termination resistors match the transmission-line impedance and suppress signal reflections at the ends of the line. For an RS485 bus requiring termination, a resistor is typically connected at each physical end of the trunk line—not at every transmitter.

The characteristic impedance of common twisted-pair cable is approximately 120Ω, so 120Ω termination resistors are often used in practice. The actual resistance should match the cable’s characteristic impedance and device requirements. If the master or end-of-line transmitter has a built-in termination resistor, check its switch setting to avoid duplicate parallel termination.

Under some conditions, a short line at a low baud rate may communicate without termination, but this should not be treated as a universal practice for all installations. For long lines, high communication rates, or lines with reflections, configure termination according to the device and cable requirements and assess the results through measurement.

Guidance for Different Locations

The termination resistor should be connected across the two differential communication wires at the corresponding end, not between a communication wire and the power supply. If the master is located in the middle of the bus, the two termination resistors should generally be installed at the physical ends of the trunk line, rather than automatically at the master.

After disconnecting power and confirming that measurement is appropriate, a multimeter can be connected across the two differential bus wires for an auxiliary check. Two 120Ω resistors in parallel have a theoretical resistance of approximately 60Ω. However, internal device circuits, bias resistors, and other connections may affect the reading, so this value alone cannot confirm that the line is operating normally.

Termination matching and bus idle-state biasing are separate matters. Bias circuitry helps establish the idle state and may already be integrated into the master or another device. Check the manufacturer’s requirements before configuring it to avoid additional loading from biasing at multiple locations.

3. How Should Addresses Be Assigned to Multiple Devices?

If the transmitters use Modbus RTU, each slave on a bus polled by the same master must have a unique address. The commonly used unicast address range is 1–247, while address 0 is generally used for broadcasts. Refer to the device’s protocol manual for its supported address range and broadcast behavior.

It is recommended to set each device’s address individually before connecting it to the bus, and to record the installation location, measurement point name, and address together. For example, assign addresses 1, 2, and 3 to the transmitters at the inlet, outlet, and backup pipeline, respectively, then use the same records to configure the measurement points in the master.

Addresses can be set using DIP switches, a keypad menu, or the manufacturer’s configuration software; the method varies by model. After changing an address, save the setting as instructed in the manual. Power-cycle the device if necessary, then read it individually using the new address to confirm that the setting has taken effect.

Example Communication Parameter Configuration

The table above is provided as a configuration example and does not represent the factory default settings for all pressure transmitters. Before wiring, check each item against the device manual and confirm that the serial port format in the master software matches the device.

New devices sometimes share the same factory-set address. If multiple devices with the same address are connected at the same time, a master request may cause multiple slaves to respond simultaneously, resulting in data conflicts. Isolate the devices one at a time, change and verify their addresses, and only then connect them to the bus.

A successful read does not necessarily mean the measured value is correct. Also check the register number, data byte order, scaling factor, and pressure unit, and compare the communication value with a field instrument or known operating conditions.

4. How to Troubleshoot Communication Problems Step by Step

If none of the devices can communicate, first check the master serial port, power supply, communication-wire polarity, and common parameters. Then check for an open or short circuit on the bus, and verify that the termination and bias settings meet system requirements. If only one device has a problem, check its address, drop line, and wiring terminals first.

If communication is intermittent or data-check errors occur, note whether the fault coincides with motor start-up or shutdown or variable-frequency drive operation. Also check the shield, grounding, cable routing, loose connectors, and drop-line length. If necessary, use an oscilloscope or communication analyzer to inspect the waveform and messages.

If connecting a device makes the entire bus unstable, first disconnect power and isolate that device, then check its A and B wires, internal termination resistor, and power supply status one by one. Test each device individually as it is reconnected to help locate the problem. Avoid repeatedly changing all parameters while a fault is present.

Distinguishing RS485 from HART When Selecting a Product

When selecting equipment for a pressure measurement project, also consider the medium, range, process connection, and environmental conditions. For differential pressure measurement, you can review the DX3351DP High-Accuracy Intelligent Differential Pressure Transmitter, a Domestic Capacitive Pressure Transmitter, which provides a two-wire 4–20mA signal with superimposed HART digital communication and supports zero, span, and damping adjustments.

Note that HART and RS485 are not the same communication interface. The product’s HART signal cannot be used directly as an RS485 bus. If a project requires connection to a unified RS485 master, first confirm with the manufacturer the available communication conversion or data acquisition solution, as well as the protocol and data mapping on the master side.

For installations in the power, metallurgy, petrochemical, and other sectors, product selection should also take into account the medium-contact materials, operating pressure, temperature, and installation conditions when confirming the specific configuration. For hazardous locations, verify the explosion-proof markings, certificates, and installation requirements for the actual ordered model.

Final Checks Before Commissioning

Check the termination at both ends of the bus, the addresses and communication parameters of all devices, and confirm that the address list matches the actual measurement points. Keep the wiring diagram and configuration records.

After testing each device individually, test multi-point polling and continuous operation. Record communication timeouts, error frames, and changes in measured values. Locate the cause of any problems before using the system for control or alarm functions.

To plan a multi-point pressure transmitter network, provide Xi’an Shenghongchuang Instrumentation Co., Ltd. with the number of measurement points, wiring distance, medium, range, and master interface information so the device communication methods and field configuration plan can be confirmed.