Agilent Technologies 78581B, Installation And Service Manual

Page: 23 / 100

Theory of Operation
2-9
Talk Tap: The talk tap carries encoded information to indicate the branch
number and to identify the “autopoll” operating mode.
Note Since the status information is block encoded, there is not a one-to-one
correspondence between status values and bits of the status words. The
condition of these bits are indicated on the status LEDs on the ACC and on
the display screens of the SDN instruments. See “SDN Status and Error
Conditions” on page 2-12.
Instrument Messages
Instrument messages originate in the instrument. These instrument
messages comprise of all the data broadcast on the SDN, such as
waveforms, derived parameters, alarm status, text messages, bed labels,
time-of-day, and many others. Instrument messages vary in length (i.e.,
number of words) but always consist of the following structure.
Instrument Delimiter: The instrument delimiter is a unique 12-bit word
(110010011111) used to mark the beginning and the end of the message.
Header: The header is always four words long (48 bits) and is used to
label the content of the message body. The second word is called the
“signature.” Listening instruments examine the signature to determine
whether they want to capture this message.
Body: The body of the instrument message contains the actual data. The
number of data words in the body depends upon the type of message being
sent.
INSTRUMENT
DELIMITER
HEADER BODY
INSTRUMENT
DELIMETER
HEADER