Complete Guide to Field Application and Troubleshooting of Mitsubishi GT15-RS2/4 Serial Communication Unit

Use an external terminal resistor: Turn both positions to OFF, and then parallel a resistor (typical value 110 Ω or 120 Ω) on the external circuit.

Common error: Connecting the terminal resistor incorrectly at the intermediate node, causing signal reflection and resulting in extremely high communication error rates. Correct approach: Only connect the terminal resistor at the head or end node.

Chapter 5 Common Communication Malfunctions and Troubleshooting Process

5.1 Fault phenomenon: GOT and PLC cannot establish a connection, and the screen displays "communication error"

Possible reasons:

The serial communication unit is not recognized by GOT (due to low OS version or incorrect installation of the unit).

Wiring error (especially the A/B polarity of RS-485 is reversed).

Communication parameters (baud rate, data bits, stop bits, parity check) do not match.

Terminal resistor configuration error.

Troubleshooting steps:

Check unit recognition: Enter the "Extended Unit Monitoring" screen of GOT (Utility menu → Extended Unit Information). If the serial unit is not displayed, upgrade the GOT system software or re plug the unit.

Checking polarity: Reverse polarity of A (+) and B (-) in RS-485 is the most common issue. Measure the A to SG voltage of the opposite end device with a multimeter, usually between 2~5V (positive relative to B). Ensure that the RDA of GOT is connected to the RDA of the other party, both of which are A+.

Verify communication parameters: Use the "Communication Parameter Setting" function of GOT (set in GT Designer 3) to ensure complete consistency with the PLC side. Special note: Many old Mitsubishi PLCs (such as the A series) use odd parity, while new devices default to even parity.

Temporary testing of terminal resistance: First disconnect all terminal resistors and test basic communication using only short wires (<10 meters). If successful, gradually increase the distance and connect the end resistor.

5.2 Fault Symptoms: Occasional communication is possible, but frequent interruptions or data errors occur

Possible reasons:

Electromagnetic interference (affecting the starting of frequency converters and high-power motors).

The shielding layer is not grounded or multiple points are grounded to form a ground loop.

The signal attenuation is caused by the cable being too long or the wire diameter being too thin.

A node in a multi site network enters a high impedance fault state.

Troubleshooting steps:

Shielding grounding optimization: The RS-485 shielding layer should be grounded at a single point (usually on the GOT or PLC side, and cannot be grounded at both ends simultaneously). Use a multimeter to check the continuity between the shielding layer and the on-site ground in the resistance range.

Increase bias resistance: Some devices lack bias resistance internally, resulting in uncertain logic state when the bus is idle. You can try adding a 470 Ω~1k Ω resistor between A-B to pull up to+5V (A) and pull down to 0V (B), or you can use a terminal resistor module with bias.

Reduce baud rate: Try reducing the communication rate from 115200 to 38400 or 19200 and observe if the interrupt frequency decreases. If it decreases, it indicates poor line quality or distance exceeding the limit.

Node by node troubleshooting: In a multi site network, disconnect all slave stations and only keep GOT and one slave station for testing. After communication is normal, restore the slave stations one by one and identify the faulty units that introduce interference.

5.3 Fault phenomenon: GT15-RS4-TE unit heats up severely or burns out

Possible reasons:

Wiring errors can cause short circuits between RS-422/485 differential signal lines or contact with external high voltage.

The terminal resistor is connected to the wrong switch combination (for example, only SW1-1 is turned on without SW1-2, causing imbalance).

On site lightning strikes or surges.

Troubleshooting steps:

Power off measurement: Use a multimeter to measure the resistance between RDA and RDB, which should be the open circuit or terminal resistance value (around 100 Ω). If it is close to 0 Ω, it indicates a short circuit in the circuit.

Check DIP switches: Ensure that SW1-1 and SW1-2 are turned on or off simultaneously, and do not allow one on and one off.

Install surge protector: Install Mitsubishi recommended surge absorber (such as RA series) at the cable inlet.

5.4 Fault phenomenon: The D-sub connector of GT15-RS4-9S is damaged

Commonly seen in rough insertion and extraction on site. Preventive measures:

Use D-sub cables with locking screws (such as 9-pin female connectors with locking components).

Regularly check if the pins inside the connector are bent. If bent, use medical tweezers to carefully correct it. However, if bent more than 3 times, it is recommended to replace the unit.

Chapter 6 Maintenance, Replacement, and Life Management

6.1 Regular inspection items

Quarterly:

Tighten all terminal screws (including installation screws and terminal block screws).

Check that there is no oxidation or corrosion at the connection of the shielding layer.

Check if the communication error count is abnormally increasing through the "self diagnosis" function of GOT.