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6.4.1 Message Types
The types of messages that are sent from the host to the instruments on the datalink are
formatted as follows:
6.4.1.1 Host to Instrument
1. Interrogate - This message requests up to 20H consecutively stored bytes,
beginning at the specified memory address location of the addressed
instrument.
01111110 EO
H
+ I.A. NUM LO ADD HI ADD LRC
2. Change - This message sends up to 20H bytes of new data to the addressed
instrument. 01111110 AO
H
+ I.A. NUM LO ADD HI ADD Data 1 XXXXXXXXX
Data N LRC
3. Change Bits - This message alters only the specified bits in the specified bytes
in the addressed instrument. (NUM = 2N)
01111110 CO
H
+ I.A. NUM LO ADD HI ADD Mask 1 State 1 XXXX Mask N
State N LRC
Table 6-2 Datalink Protocol
Symbol Description
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
It is the operation to be performed or a description of the message that follows the Com-
mand-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 as follows: Interrogate, Change, Change Bits,
Acknowledge, and Response.
NUM
Number
The number of data bytes transferred or requested. The NUM must be a range of 00-32
decimal.
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
Has a 0 in every bit position that is selected to be forced to 1.
STATE
Has a 1 in every bit position that is selected to be forced to 1.
LRC
It is the sum of all bytes Modolo 256 of the message not including the SOH character or its
own bit settings (LRC). It is written at the end of the message to ensure data was not lost in
transmission.
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