SMAR TT301, Operation, Maintenance & Instruction Manual

Page: 17 / 74

Section 2
2.1
OPERATION
The
TT301 accepts signals from mV generators such as thermocouples or resistive sensors such as
RTDs. The criterion is that the signal is within the range of the input. For mV, the range is -50 to 500
mV and for resistance, 0 to 2000 Ohm.
Functional Description-Hardware
Refer to the block diagram (Figure 2.1). The function of each block is described below.
LOCAL ADJUST
MODEM
BELL 202
POWER
SUPPLY
OUTPUT
DISPLAY
CONTROLLER
AMBIENT
TEMPERATURE
SENSOR
DISPLAY BOARD
SUPPLY
SUPPLY
MAIN BOARD INPUT BOARD
MUX
SIGNAL
CONDI-
TIONER
TC
Ohm
RTD
mV
I
S
O
L
A
T
O
R
A/D
CONVER-
TER
D/A
CONVERTER
PROCESSING UNIT
RANGES
SPECIAL FUNCTIONS
PID (OPTIONAL)
OUTPUT CONTROL
SERIAL COMMUNICATION
HART PROTOCOL
POWER
ISOLATION
Figure 2.1
– TT301 Block Diagram
MUX-Multiplexer
The MUX multiplexes the sensor terminals to the signal conditioning section ensuring that the voltages
are measured between the correct terminals.
Signal Conditioner
Its function is to apply the correct gain to the input signals to make them suit the A/D converter.
A/D Converter
The A/D converts the input signal to a digital format for the CPU.
Isolator
Its function is to isolate the control and data signal between the input and the CPU.
CPU - Central Processing Unit & PROM
The CPU is the intelligent portion of the transmitter, being responsible for the management and
operation of all other blocks: linearization, cold junction compensation and communication. The
program is stored in the PROM as well as the linearization data for the temperature sensors.
For temporary storage of data, the CPU has an internal RAM, the data in the RAM is lost if the power is
switched off, however the CPU also has an internal nonvolatile EEPROM where data that must be
retained is stored. Examples of such data are: calibration, configuration and identification data.
D/A Converter
Converts the digital data from the CPU to an analog signal with 14-bits resolution.
Output
Controls the current in the line feeding the transmitter. It acts as a variable resistive load whose value