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

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

In automated production lines, serial communication between human-machine interface (HMI) and on-site devices such as PLC, frequency converter, and instruments is the foundation of data exchange. The Mitsubishi Electric GOT2000/GOT1000 series graphic operation terminal, with its rich expansion capabilities, can flexibly connect multiple interfaces such as RS-232, RS-422, RS-485 through a dedicated serial communication unit. Among them, the GT15-RS2-9P, GT15-RS4-9S, and GT15-RS4-TE units cover point-to-point and multi-point communication scenarios, and are widely used in the renovation of old production lines, remote equipment monitoring, and multi station data acquisition.

However, serial communication is limited by factors such as wiring quality, terminal resistance matching, and shielding grounding, and on-site engineers often encounter tricky problems such as "unstable communication", "occasional disconnections", and "inability to establish connections". This article combines Mitsubishi's official technical information and years of on-site maintenance experience to systematically sort out the technical specifications, installation steps, wiring specifications, and quick troubleshooting process of these three serial communication units, providing a practical "pocket guide" for automation maintenance personnel.

Chapter 1 Model Overview and Selection Points

Mitsubishi provides three types of serial communication expansion units for the GOT2000/GOT1000 series, corresponding to different electrical interfaces and application scenarios.

Model Interface Type Connector Form Maximum Transmission Distance Typical Applications

GT15-RS2-9P RS-232 D-sub 9-pin (male) 15 meter close range connection to Mitsubishi PLC, barcode scanner, printer

GT15-RS4-9S RS-422/485 D-sub 9-pin (female) 1200 meter multi station RS-485 network, connected to frequency converters and instruments

GT15-RS4-TE RS-422/485 terminal block (pluggable) 1200 meters for industrial environments requiring high reliability wiring, convenient for shielded cable handling

Key points of selection decision:

If the communication distance is ≤ 15 meters and only a single device needs to be connected, choose GT15-RS2-9P (RS-232). Note that this unit cannot be used for multi station networks.

If long-distance (up to 1200 meters) or multi station (1: N) communication is required, RS-422/485 type must be selected. Among them, GT15-RS4-9S is convenient for using standard D-sub cables, while GT15-RS4-TE is suitable for on-site self-made cables that require good shielding grounding.

Special restriction: GT15-RS4-TE does not have RS/CS hardware flow control signals, so it cannot be directly connected to specific PLCs (some older models) that rely on this signal. Please refer to the Mitsubishi Connection Manual before selecting.

Chapter 2 Detailed Explanation of Technical Specifications

2.1 Performance Parameters

All three units share the following communication parameters:

Transmission speed: 115200/57600/38400/19200/9600/4800 bps (configurable)

Synchronization method: Asynchronous (start stop)

Error detection: parity check, sum check

Internal current consumption (5VDC):

GT15-RS2-9P：0.29A

GT15-RS4-9S：0.33A

GT15-RS4-TE：0.30A

Weight: Approximately 0.09kg

Attention: When using a serial communication unit, the GOT needs to install a standard monitoring OS and communication driver, with version GT Modeler 2 Version 2.15R or higher. The old version firmware cannot recognize the unit, which can result in an error of 'expansion unit not detected'.

2.2 Environment and General Specifications

The general specifications of the serial communication unit are consistent with the installed GOT host. For example, when installed on the GT15 series, the working temperature range is 0-55 ℃, the storage temperature is -20~60 ℃, and the humidity is 10-90% RH without condensation. The impact resistance follows the IP rating of the GOT host (usually IP67F or IP20F). In sites with severe dust, oil mist, or vibration, additional protective covers are required.

2.3 Terminal block specifications (GT15-RS4-TE only)

GT15-RS4-TE adopts a pluggable terminal block for easy cable maintenance:

Screw torque: 0.20~0.25 N · m (terminal block fixing screw); 0.5~0.6 N · m (wire clamping screw)

Applicable wire: 0.08~1.5 mm ²

Recommended tool: Screwdriver that meets DIN 5264 standard, with chrome plated tip

Shielding layer treatment: Comes with plate type crimping terminals for connecting shielding braided mesh

Chapter 3 Installation and Expansion Structure

3.1 Unit installation steps (taking GT1575 as an example)

The serial communication unit is installed on the expansion interface on the back of the GOT. The installation process does not require special tools, but the following sequence must be strictly followed:

Power off: Turn off GOT and all external power sources of the system (completely cut off all phases).

Remove the cover plate: Remove the expansion unit cover on the back of the GOT (one or two, depending on the installation position).

Align insertion: Align the expansion interface of the serial communication unit with the expansion connector of the GOT, and push it in parallel until it feels fully engaged.

Tighten screws: Use a Phillips screwdriver to tighten the two mounting screws on the unit. The torque range is 0.36~0.48 N · m. Being too tight can damage the plastic studs, while being too loose can lead to poor contact.

Multi level expansion: If additional expansion units (such as Ethernet or bus connections) need to be installed after the already installed serial unit, the connector cover and sticker on the front end of the unit need to be removed. If the expansion is not continued, the attached anti-static sticker should be pasted to cover the top of the screw, and the connector cover should be kept to prevent foreign objects from entering.

Warning: When disassembling the serial communication unit, it is necessary to hold the "PULL" indicator on the side of the unit and pull it out horizontally. Do not tilt or shake it, otherwise the internal connector pins will be damaged.

3.2 Terminal Block Installation (GT15-RS4-TE only)

The terminal block of GT15-RS4-TE is pluggable. It is recommended to unplug the terminal block from the unit before wiring:

Align the protrusion of the terminal block socket with the guide groove on the unit and insert it vertically.

Tighten the fixing screws on both sides of the terminal block (torque 0.20~0.25 N · m) to prevent vibration loosening.

Practical tip: In a multi expansion layer structure, there is no need to remove all expansion units from the GOT. Simply remove the terminal block of the target GT15-RS4-TE for wiring or replacement, greatly reducing downtime.

Chapter 4 Wiring Specification and Terminal Resistance Configuration

Good wiring is the cornerstone of stable serial communication. The following wiring instructions are provided for three different models.

4.1 GT15-RS2-9P (RS-232) Wiring Points

Connector: D-sub 9-pin male connector, compatible with standard straight or crossover cables (depending on the definition of the end device).

Maximum distance: 15 meters, it is recommended to use shielded RS-232 dedicated cables (such as Mitsubishi GT01-C □□□□ -9P series).

Foot position definition (reference standard RS-232 DTE):

2-pin: RXD (receive data)

3-pin: TXD (sending data)

5 feet: SG (signal ground)

Other pins (such as 4, 6, 7, 8) can be disconnected in simple three wire communication.

Shielding treatment: The shielding layer is only grounded on the single end of the GOT side to avoid ground circulation.

Common faults: Sometimes there is no communication and accompanied by garbled code → Check if the transmission and reception are cross connected (the TXD of GOT should be connected to the RXD of the device). If the peer device is DCE (such as a modem), a direct line is required.

4.2 GT15-RS4-9S (RS-422/485 D-sub) wiring

Connector: D-sub 9-pin female connector. Recommend using Mitsubishi specific cables (such as GT01-C □ R4-25P).

Signal allocation (typical RS-422/485 4-wire system):

RDA (receive A+), RDB (receive B -)

SDA (send A+), SDB (send B -)

SG (Signal Ground)

ERA, ERB (optional RS-422 full duplex control, generally not connected)

Two wire half duplex: RDA and SDA need to be short circuited, RDB and SDB need to be short circuited (there is no automatic switching inside the unit).

Terminal resistor: Control the built-in 100 Ω resistor through DIP switch (positions 1 and 2 of SW1). Only devices located at the physical end of communication lines need to have terminal resistors connected.

4.3 GT15-RS4-TE (terminal block type) wiring

This is the most flexible model on site and also a high-risk area for malfunctions. The correct wiring steps are as follows:

4.3.1 Wire preparation

Peel off the cable sheath by 7mm (0.28 inches) to expose the copper core.

If using multiple strands of flexible wire, it is recommended to crimp U-shaped or tubular cold pressed terminals to avoid loose wire short circuits.

4.3.2 Shielding layer connection (key!)

The attachment provides a "plate type solerless terminal" for fixing the shielding braided mesh:

Fold the shielding braided mesh of RS-422/485 cable backwards to expose a section of approximately 10mm.

Press the shielding net under the board terminal and tighten it with wire clamps.

Connect the board terminal to the ground terminal (marked as FG or SLD) on the terminal block.

Attention: The shielding layer cannot be directly screwed onto the terminal screw, otherwise it is easy to loosen.

4.3.3 Wire Connection

Connect the wires according to the markings on the terminal block (such as RDA, RDB, SDA, SDB, SG).

Tighten the wire screw to a torque of 0.5~0.6 N · m. Too small can easily loosen, and too large pressure can break the wire core.

The insertion direction must be horizontal (parallel to the long side of the terminal block). If inserted vertically, it is prone to detachment after vibration.

4.3.4 Terminal Resistance Setting (DIP Switch)

The DIP switch of GT15-RS4-TE is located on the side of the unit (SW1, two position switch):

Built in terminal resistor ON: Turn both SW1-1 and SW1-2 to the ON position (connect 100 Ω).

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.

Every year:

The measured RS-485 differential voltage (A-B) should be -0.2V~+0.2V (without bias) or stable above+0.8V (with bias) when communication is idle.

Observe the signal waveform with an oscilloscope, and the rising and falling edges should be smooth without overshoot.

6.2 Unit replacement steps (quick on-site replacement)

When it is determined that there is a hardware failure in the serial communication unit (such as internal short circuit, connector breakage), replace it according to the following process:

Backup parameters: Upload project data of GOT through GT Designer 3 and record communication settings (baud rate, station number, protocol).

Power off: Turn off GOT and external 24V power supply.

Dismantling: Unscrew the installation screws and pull out the old unit in the direction of "PULL".

Set up a new unit: If the new unit is GT15-RS4-TE or GT15-RS4-9S, set the DIP switch (terminal resistor) to the same state as the original unit in advance.

Install new unit: Align the expansion interface and press in, tighten the screws (torque 0.36-0.48 N · m).

Reconnect the external line: GT15-RS4-TE needs to plug the original terminal block back in (pay attention to the direction) and check the shielded wiring.

Power on test: Enter the GOT communication monitoring screen and confirm that the error count does not continue to increase.

Fill in maintenance records: record the replacement date, unit serial number, and fault symptoms.

Notes:

After replacement, the same model as the original must be used (for example, GT15-RS4-9S cannot replace GT15-RS4-TE because although the electrical characteristics are the same, the connector form makes it impossible to directly replace it).

If the GOT firmware version is lower than the requirement, the new unit will also not be recognized - the GOT system needs to be upgraded synchronously.

Chapter 7 EMC, RoHS and Environmental Compliance

The Mitsubishi serial communication unit complies with the following international standards:

EMC Directive (89/336/EEC): System level installation must be carried out in accordance with the "EMC and Low Voltage Directive" guidelines (such as using shielded cables and proper grounding).

Low Voltage Directive (73/23/EEC): The unit itself is a Safety Ultra Low Voltage (SELV) circuit, but the external 24V power supply connected to the system must comply with safety standards.

China RoHS: According to the Management Measures for the Restriction of Hazardous Substances in Electrical and Electronic Products, the circuit board components in the unit contain lead (Pb), but the resin shell and metal parts meet the standards. When disposing of waste, it must be classified and recycled according to industrial waste.

CE mark: printed on the GOT host nameplate, not on the serial unit, but the overall system must meet EMC requirements.