Micromod Micro-DCI: 53IT5100B Micro-DCI 4-Channel Indicator Totalizer - Manual (Page 59)

53IT5100B Indicator/Totalizer
INSTRUCTION MANUAL
55
Protocol
The Datalink protocol requires the host 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 addressed 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).
Message Types
The types of messages that are sent between the host and the Datalink network instrument are formatted as
follows:
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 E0H + 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 A0H + 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 C0H + I.A. NUM LO ADD HI ADD Mask 1 State 1 XXXX Mask N State N LRC