B.1.2 protocol, Table b-2. message field definitions – Micromod Micro-DCI: 53SL5100A Single Loop Controller User Manual
Page 135
Single Loop Process Controller Instruction Manual
Communications 127
B.1.2 Protocol
The Data link protocol requires the host or SUPERVISOR-PC to initiate alltransactions. There are two
basic categories for all of the Datalink message types:
•
Interrogate, used to read data from an addressed instrument.
•
Change, used to alter a value in an addressed instrument.
The addressed instrument decodes the message and provides an appropriate response. The protocol def-
initions for the Datalink message types are provided in
.
Table B-2. Message Field Definitions
Symbol
Title
Definition
SOH
Start of Header
This character, 7E, denotes the beginning of a message.
I.A.
Instrument Address
The address of the instrument responding to the transaction. It
must be within a range of 00-1F (00-31 decimal).
CMD
Command
Is the operation to be performed or a description of the
message that follows the Command-I.A. byte. The Command-
I.A. byte has two fields:
•
the Command field (3 bits), and
•
the I.A. field (5 bits)
There are five commands:
Interrogate
Change
Change Bits
Acknowledge
Response
The commands are covered in
.
NUM
Number
The number of data bytes transferred or requested. The NUM
must be in a range of 00-32.
LO-ADD
Lower Address Bits
The least significant 8 bits of a 16 bit instrument address.
HI-ADD
Higher Address Bits
The most significant 8 bits of a 16 bit instrument address.
DATA
An 8 bit data byte.
XXXX
Represents a variable number of data bytes.
MASK
An 8 bit byte where each bit, called a flag, is dedicated to an
event that is permitted or prohibited, depending on the flag
setting. If the flag is set to 0, the event is permitted. If the flag is
set to 1, the event is prohibited.
STATE
Represents the bit settings of a particular byte: which bits are
set to 1 and which bits are set to 0.
LRC
Longitudinal
Redundancy
Character
Is a character written at the end of the message that represents
the byte content of the message and is checked to ensure data
was not lost in transmission. It is the sum of all bytes Module
256 of the message not including the SOH character or its own
bit settings (LRC).