SDN Data Structure — System and Instrument Messages
2-8
Autopoll Mode
In the event that the ACC goes down (loses power, for example), all
instruments on the same branch continue to communicate with each other
in the autopoll mode. Autopoll means that the first instrument (closest to
the wall box) sends sync taps and talk taps to itself and the other
instruments on the branch. The timing is exactly the same as normal SDN
operation. When the ACC resumes proper operation, the instrument
automatically leaves the autopoll mode and returns to SDN
communication.
SDN Data Structure — System and Instrument Messages
Digitized information travels serially on the SDN in 12-bit words. These
words are grouped together in a string to form complete messages. The
SDN uses two types of messages: system messages which originate in the
ACC or in the first instrument on branch when in the autopoll mode and
instrument messages that originate in the instruments.
System Messages
System messages originate in the ACC. System messages consist of three
separate 12-bit words and have the following structure:
Flush:
The flush is a unique sequence of 12 bits (101010101010) that are
used to clear the line of capacitive charging to ensure reliable detection of
logic levels. The flush word always follows any silence on the SDN bus.
System Delimiter:
The system delimiter is another unique sequence of 12
bits (100000111111) that are used to help the instruments’ transceivers
recognize the system message and establish synchronization.
System Status Word:
The system status word consists of either a sync tap
or a talk tap.
Sync Tap:
The sync tap carries encoded information to indicate whether
the current poll cycle is the first poll cycle in the system cycle (master poll
cycle), to flag “fire axe” and “poll overflow” error conditions (see “SDN
Status and Error Conditions” on page 2-12), and to identify the “autopoll”
operating mode (see “Autopoll Mode” on page 2-8).
FLUSH
SYSTEM
DELIMITER
SYSTEM STATUS
WORD
Summary of Contents for 78581B
Page 46: ...ACC Maintenance 4 2 ...
Page 54: ...Installation Responsibilities Customer and Agilent Technologies 6 4 ...
Page 90: ...Procedures 8 28 ...
Page 98: ...Contacts 10 6 ...
