Manual

B.1.2 PROTOCOL
The Datalink protocol requires the host or SUPERVISOR-PC to initiate all transactions. There are
two basic categories for all of the Datalink message types:
Interrogate
- used to read data from
an addressed instrument, and
Change
used to alter a value in an addressed instrument. The ad-
dressed instrument decodes the message and provides an appropriate response. The protocol
definitions for the Datalink message types are provided in Table B-2.
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, listed as follows:
Interrogate
Change
Change Bits
Acknowledge
Response
The commands are covered in Section B.1.3, Message Types.
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 Modulo 256 of the message not including the SOH
character or its own bit settings (LRC).
Appendix B. Communications
B-3