FC 76 - Thermocouple Temperature (NAMM02 - IMAMM03)

The thermocouple temperature function code provides thermocouple temperatures at the termination device to the NAMM02 and IMAMM03 Analog Master Modules. Use this function code only for monitoring; it has no specifications. All temperatures are degrees Celsius.

 

Besides termination device thermocouple temperatures, function code 76 also makes available the system trend time and module revision levels in blocks 94 through 97. Blocks 90 through 97 can only be accessed via a read point or read block message on the module bus (monitor on the configuration and tuning module).

 

Function code 76 resides in fixed blocks 90 through 97.

 

 

Outputs:

 

Blk

Type

Description

90

R

RTD pair A thermocouple TU temperature

91

R

RTD pair B thermocouple TU temperature

92

R

RTD pair C thermocouple TU temperature

93

R

RTD pair D thermocouple TU temperature

94

R

System trend time, hours

95

R

System trend time, minutes

96

R

System trend time, seconds

97

R

Module Revision Level

 

 

 

76.1  Explanation

 

The termination device measures the thermocouple temperatures to insure accurate reporting of field thermocouple temperatures. Since the connection to the termination device creates a junction of dissimilar metals, an additional voltage is created and subtracted from the one produced by the thermocouple,leading to a false value for the thermocouple temperature.

 

The NAMM02/IMAMM03 takes the termination device thermocouple temperature provided by this function code, converts it to a voltage signal internally, and adds it to the initial voltage signal to correct the value before it is converted to a temperature for output from the module.

 

The analog master module accepts inputs from eight I/O modules through termination devices. Each termination device can accommodate two I/O modules. The I/O module inputs feed into an RTD pair that measures the ambient temperature of the air surrounding the termination device. The use of RTD pairs are for temperature correction of thermocouple inputs.

Termination devices are usually daisy chained together as shown in Figure 76-1, with RTD pairs named according to termination device position on the daisy chain. Table 76-1 shows the correspondence between RTD pairs and block numbers.

 

 

 

 

Table 76-1. Correspondence Between RTD Pairs and Block Numbers

 

Block Number

RTD Pair

90

A

91

B

92

C

93

D