Theory Of Operation; Basic Operation - Honeywell STT25D Operator's Manual

3.1

Basic Operation

As shown in Figure 2, the transmitter is powered via the 2 wire, 4-20 mA signal
connected to the + and - terminals on the output side of the module. Inputs are
sampled at a rate of 2 times per second and digitized by the A/D converter. Data is
then transferred across the galvanic isolation interface (both power supply and signal
interface of the A/D converter are galvanically isolated), compensated for cold
junction or resistance lead length. The process value is post read validated for
sensor wiring and signal integrity against reference values. Digital data is then
linearized and ranged to the lower and upper range values held in non volatile
memory and converted back to an analog signal. Any configuration changes are held
in non volatile memory so that they are secured against power failure. If a custom
configuration was not specified, the data programmed into the non volatile memory of
the unit at the manufacturing location is the default shipping data shown below.
Tag I.D.
Sensor type
Sensor fault detection
Latching
Line filter
Output type
Write protect
Password
Damping
LRV (Lower Range Value)
URV (Upper Range Value)
Output mode
Digital DE conf.
The data can be configured in the field by connecting a Communicator across the 4-
20 mA wires or a PC. The fail-safe link between the + and - terminals determines
where the output will drive when the STT250 detects an open sensor input or internal
failure. The unit will drive upscale to 21.8 mA when the fail-safe link is in the U position
and downscale to 3.8 mA in the D position.
The output of the STT250 can be 4-20mA analog (DE or HART) or digital DE protocol.
The digital DE output is used primarily with Honeywell's control system where it
improves performance by avoiding conversion to/from an analog signal and offers full
database integration of field transmitters with the central control system
(1) To protect the integrity, write protect is software configurable and accessible
through a password. The fall-back password is an algorithm based on the unit serial
number. If the password is lost, contact your regional Technical Assistance Center
(TAC) with the unit serial number.
Smart Temperature Transmitter STT250 – Operator Manual

3. THEORY OF OPERATION

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mV
ON
Disabled
50 Hz (Amiens) - 60 Hz (Phoenix)
Linear
(1)
Disabled
0000
0 second
0mV
45mV
Analog (DE or HART)
6 byte/Single rng.-S V
.
3