New MITSUBISHI PLC Module Q64TDV-GH In Box Q64TDVGH

Mitsubishi Q64TDV-GH | MELSEC-Q Series Channel-Isolated Thermocouple / Millivolt Input Module — 4 Channels, 16-Bit, Disconnection Detection, Multiple Thermocouple Types, Scaling

Part Number: Q64TDV-GH

Manufacturer: Mitsubishi Electric Corporation (Japan)

Product Line: MELSEC-Q Series — Temperature / Analog Input Modules

Function: Channel-isolated thermocouple and millivolt analog input module; converts thermocouple signals directly to temperature values or micro-voltage signals to 16-bit digital values

Input Channels: 4 (fully channel-to-channel isolated)

Compatible with Redundant Systems: Yes (Q-series redundant CPU systems)

Configuration Tool: GX Developer / GX Works2 (intelligent function module parameter setting)

Status: Active product (commercialised)

Applications: Industrial furnace temperature monitoring, process reactor temperature measurement, thermocouple-based temperature control, precision mV signal acquisition from load cells and bridges, multi-zone temperature logging

Overview

The Mitsubishi Q64TDV-GH combines two fundamentally different measurement tasks into one four-channel MELSEC-Q module: direct thermocouple temperature measurement with cold junction compensation and linearisation built in, and general-purpose millivolt analog input for applications where the raw sensor output is a low-level voltage that standard ±10V or 4–20mA input modules cannot handle without external signal conditioning.

The "GH" designation identifies this as the channel-isolated, high-accuracy variant — the version where every input channel has its own independent ground reference, preventing the inter-channel interference that occurs in non-isolated modules when thermocouples at different measurement points share a common reference.

Thermocouples remain the dominant temperature sensor in industrial furnace, kiln, and process reactor applications because of their temperature range, durability, and established reliability.

A Type K thermocouple measures from approximately −200°C to +1370°C, a Type B handles up to +1820°C for very high-temperature furnaces, and the R and S types cover platinum thermocouple applications in precision high-temperature work.

The Q64TDV-GH handles all of these natively — the engineer selects the thermocouple type per channel through the module switch settings, and the module applies the correct IEC linearisation table internally.

The CPU receives a value directly representing temperature in tenths of a degree Celsius; the conversion mathematics happen inside the module without any ladder program calculation required.

The millivolt input mode extends the module's utility beyond thermocouples to any signal source generating a low-level voltage in the ±25mV range (full-scale) or narrower sub-ranges.

Load cells, strain gauge bridges, precision resistive dividers, and some specialty transducers output signals in the millivolt range — signals that a conventional ±10V or 4–20mA analog input module cannot accept without a signal conditioning amplifier. 

The Q64TDV-GH eliminates that amplifier, connecting directly to the sensor output and digitising the millivolt signal at 16-bit resolution.

Key Specifications

Channel Isolation — Why It Matters for Thermocouple Measurements

The full channel-to-channel isolation in the Q64TDV-GH is not just a feature for demanding applications — it is an engineering necessity for accurate thermocouple measurements across an industrial plant.

In a process facility, thermocouples are installed at physically separate measurement points: different vessels, different piping sections, different zones of a furnace. 

Each thermocouple's negative terminal is at the temperature of the process junction, and because thermocouples are usually installed without deliberate grounding, each may be at a slightly different electrical potential relative to the cabinet's reference ground.

In a non-isolated multi-channel input module, all channels share a common terminal.

When thermocouples at different potentials connect to a shared-common module, current flows through the common terminal between channels — the measurement current for one channel passes through the reference for another. 

This inter-channel crosstalk corrupts each channel's reading with an error proportional to the common-mode voltage difference between channels, an error that varies with process conditions and is extremely difficult to diagnose.

Full channel isolation eliminates this by providing each channel with its own completely independent reference — no shared terminal, no current path between channels. Each thermocouple's circuit is completely contained within its own channel's isolated measurement circuit.

Channels that are at very different potentials coexist without affecting each other's measurement, and grounded thermocouples (where the measurement junction is in physical contact with a grounded metal structure) can be mixed with floating thermocouples in the same module without generating crosstalk errors.

Disconnection Detection and Fault Output Handling

One of the most practically important features of the Q64TDV-GH is its disconnection detection — the module actively monitors whether each thermocouple input is receiving a valid signal or has developed an open circuit.

Thermocouple cables are physically fragile, break from mechanical vibration and repeated thermal cycling, and are exposed to corrosive environments that damage terminals and insulation. 

A disconnected thermocouple is a common field fault, and the consequences of an undetected disconnection can range from process inefficiency to equipment damage if the temperature reading silently drops to a misleading value.

When the Q64TDV-GH detects an open input (no thermocouple signal present), it responds according to the configured disconnection output selection:

UP scale forces the reported value to the maximum of the measured temperature range plus 5% — a high alarm value that immediately triggers high-temperature alarm logic in the CPU program, alerting the operator to investigate.

DOWN scale forces the value to the minimum of the range minus 5% — a low alarm value useful where falling below a minimum temperature is the critical failure mode.

Previous value holds the last valid reading — appropriate for applications where a brief transient disconnection should not trigger an alarm, but where sustained disconnection will eventually be detected by other monitoring logic.

Given value sets a specific user-defined value — for applications where the disconnection state should be represented by a specific setpoint in the control logic.

The appropriate response depends on the safety logic of the process: in heating applications where losing temperature feedback means the heater continues at full power, UP scale is usually the correct choice because the high alarm immediately shuts down the heater. In cooling applications, DOWN scale triggers the alarm response appropriate to loss of temperature control.

Built-in Scaling for Direct Engineering Unit Output

The Q64TDV-GH's built-in scaling function converts the raw digital temperature or millivolt value into any user-defined engineering unit range before storing it in the buffer memory for the CPU to read. Rather than having the CPU program perform the scaling calculation every scan cycle, the engineer configures the input and output range limits in the module's parameter settings, and the module performs the linear conversion internally.

For process control applications where the temperature reading needs to drive a PID controller using a percentage of span (0–100%) rather than absolute temperature, or where the engineering display should show the temperature in Fahrenheit while the module measures in Celsius, the scaling function handles the conversion transparently.

The CPU reads the already-scaled value directly — no ladder math required for the unit conversion.

FAQ

Q1: Can each of the Q64TDV-GH's four channels be set to a different thermocouple type, or must all four use the same type?

Each channel has its own independent input type setting, configured through the intelligent function module switch settings in GX Developer or GX Works2.

Channel 1 can be set for Type K, channel 2 for Type J, channel 3 for millivolt direct input, and channel 4 for Type T — all simultaneously active with their own linearisation applied. 

This flexibility makes the Q64TDV-GH suitable for installations where different sensor types are used in different measurement locations without requiring separate modules per thermocouple type.

Q2: What is cold junction compensation, and why is it included in the Q64TDV-GH?

A thermocouple generates a voltage proportional to the temperature difference between its measurement junction (at the process) and its reference junction (where the thermocouple wire connects to the module's terminals).

To obtain the actual process temperature, the reference junction temperature must be known and added to the voltage-derived temperature. Cold junction compensation is the process of measuring the temperature at the module's terminal block and adding this to the thermocouple voltage calculation.

The Q64TDV-GH includes an internal temperature sensor at its terminal block for this purpose, performing the cold junction compensation automatically.

Without it, the reported temperature would be accurate only if the module's terminals were at 0°C — which is never the case in a control cabinet.

Q3: The Q64TDV-GH conversion speed is 40ms per channel. Is this fast enough for furnace temperature control?

For most industrial furnace and process control applications, 40ms per channel is entirely adequate.

Thermal processes in furnaces, kilns, and reactors have time constants ranging from seconds to minutes — the temperature changes so slowly relative to the 40ms sample rate that even a 4-channel configuration updating every 160ms provides far more measurement bandwidth than the process dynamics require. 

For truly fast thermal events (microsecond-scale thermal transients or high-speed calorimetry), the Q64TDV-GH is not the appropriate sensor interface — but these applications are not the target for standard thermocouple measurement modules.

Q4: Can the Q64TDV-GH be used in Mitsubishi's redundant CPU system configuration?

Yes. The Q64TDV-GH is compatible with MELSEC-Q redundant CPU systems.

In a redundant system, the module operates normally with both CPUs sharing its data; during a CPU switchover caused by a CPU fault, the module continues converting and reporting data without interruption. 

The redundant system configuration must be set up correctly in GX Developer/GX Works2 according to the redundant system application requirements, and the Q64TDV-GH's parameter settings must be identical in both CPUs' configurations to ensure seamless switchover behaviour.

Q5: How is the Q64TDV-GH wired for millivolt input from a load cell or strain gauge bridge?

For millivolt direct input, the signal source (load cell output, bridge amplifier output, or similar) connects between the CH□ V+ and V– terminals for the configured channel.

The cable should be shielded twisted-pair, with the shield grounded at one end only (typically the module end) to prevent common-mode noise coupling. 

The signal cable must be routed at least 100mm away from main power circuits and AC control lines to avoid induction.

Because the millivolt input is extremely sensitive (full scale is ±25mV), even small induced voltages from nearby power wiring can appear as measurement error — cable routing discipline and proper shielding are essential for accurate millivolt measurements.