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Channel Active Each channel can be configured as either active or inactive. If a channel is inactive, it is still sampled from the A/D. The filtering, scaling, calibration, and alarm checks are omitted for that channel, and a value of 0 is returned to the BIU. The reference parameter for the analog input data returns the byte length and is independent of the number of active channels. If a channel is active, and the configured analog input data length is not long enough to accommodate the data for a particular channel, the data for that channel is still processed, since the response to a group command may be used to transmit that channel’s data to the BIU. Low Alarm Limit and High Alarm Limit Each input channel can have a low alarm limit and a high alarm limit. If an input reaches one of its limits, the module reports the actual value and sends the appropriate diagnostic bit in the BIU’s discrete input (I) table.. Alarms do not stop the process or change the value of the input. Alarm limits can be set anywhere over the dynamic range of the signal. The range for each is – 32,768 to +32,767. The high alarm limit must be greater than the low alarm limit. If alarm reporting is not wanted, alarm limits can be set beyond the dynamic range of the signal so they will never be activated. Cold Junction Compensation The Thermocouple module provides four methods of Cold Junction Compensation.
No Cold Junction Compensation: This is used for millivolt inputs or if cold junction is maintained at 0 degrees C.
Remote Cold Junction Compensation: With this option, cold junction is measured externally and provided to the module by the BIU, using its %AQ output references. If the module has multiple thermocouples that are configured for remote compensation, the same compensation value must be used by each.
Fixed Cold Junction Compensation: This option uses a fixed compensation value which is provided as part of the module configuration.
Local Cold Junction Compensation: The best way to provide local compensation is with a Thermocouple Terminal Block. (IC670CHS004), which has two built-in thermistors. It shields thermocouple connections from module heat. If local compensation is selected and a Thermocouple Terminal Block is not used, separate thermistors must be installed directly at the I/O Terminal Block, using the Thermistor (+) and Thermistor (–) terminals. The thermistor must be BetaTHERM part # 8.5K3A4 (or equivalent) from BetaTHERM Corp., 910 Turnpike Rd., Shrewsbury MA, 01545 (ph: 508 842-0516, FAX: 508 842-0748) Note: If local compensation is selected but a Thermocouple Terminal Block or local thermistors are not used, erroneous temperatures may be reported. Range Selection The module can be configured for any of six different millivolt ranges (+/–): 19.5mV, 39mV, 78.125mV, 156.25mV, 312.5mV, and 625mV. All but the last provide input readings in hundredths of millivolts. For the 625mV range, inputs are in tenths of millivolts.