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DESCRIPTION AND OPERATION
SMART TRANSMITTER COMMUNICATION
I-E96-304A
2 - 5
The operator can enter one command from each column
through the MFP during a failure. The ASI stores only the last
command entered of each set. The transmitter receives this
last command when the parameters download from the ASI.
Data Byte
The point data is two 8-bit bytes, each bit of data represents
one input. The bit value reflects the state of that input, either
open (logic 1) or closed (logic 0).
Status Byte
The status byte makes sure the slave expander bus data and
the master module setup data are correct. The ASI sends a sta-
tus byte to ensure correct data transfer. The status byte com-
pares the sum of the bytes sent to the sum of the bytes
received. The master module reads the status byte and com-
pares it to an expected value. If a mismatch occurs, it flags the
error and marks the point as bad quality.
Security Byte
When a smart transmitter is operating with the ASI in a control
system, transmitter operating parameters should not be
changed except through the ASI. To prevent changes, the ASI
sends a security byte at the beginning of each communication.
The security byte triggers the smart transmitters to take com-
mands only from the ASI. The security byte locks out com-
mands, such as reranging, from any other interface device.
These devices (smart transmitter terminals) connect to the
transmitter current loop.
Communication Lockout Byte
In normal operation, the ASI sends the lockout byte once each
minute to the smart transmitters when the ASI checks trans-
mitter status. If the ASI does not send a byte within a two
minute period, the transmitter unlocks itself, and accepts com-
mands from other sources on the current loop. You can unlock
transmitters to allow communication from other sources. To do
this, set specification 1 in Function Code 133, smart transmit-
ter definition. This lockout feature prevents unwanted trans-
mitter configuration changes. To enter changes, connect a STT
terminal to the plug in the front of the ASI module. Changes
also can be entered through another interface device connected
to the transmitter current loop while the system is operating.