What this thermocouple input module does
The Q68TD-G-H02 is an 8-channel thermocouple input unit with channel-to-channel insulation, purpose-built for the Mitsubishi MELSEC Q Series PLC platform. It reads eight thermocouple inputs plus a cold-junction compensation channel, converting temperature to a 16-bit signed binary value (-2700 to 18200) that maps to the PLC's analog input registers. Resolution varies by thermocouple type: 0.3 °C for types B, R, S, and N; 0.1 °C for types K, E, J, and T. The sampling cycle is 320 ms for all eight channels, with a conversion speed of 640 ms per eight channels — the module updates the full set of readings every 640 ms.
Isolation and wiring constraints
Channel-to-channel insulation means each thermocouple input is isolated from the others by a transformer, with a withstand voltage of 1000 Vrms AC between channels for one minute. Between the thermocouple input and the PLC power supply, the insulation is also transformer-based, rated 500 Vrms AC for one minute. Insulation resistance is 10 MΩ or more at 500 VDC. External wiring connects via a 40-pin connector (A6CON1, A6CON2, or A6CON4, sold separately). The module occupies 16 I/O points (intelligent module assignment) and draws 0.65 A from the 5 V backplane supply.
Thermocouple compatibility and diagnostics
The module complies with JIS C1602-1995 and IEC 60584-1 (1995) / IEC 60584-2 (1982) thermocouple standards, covering types B, R, S, N, K, E, J, and T. Each channel has independent disconnection detection — if a thermocouple wire opens, the module flags that channel without affecting the others.