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EOI, End or Identify, is used in conjunction with ATN: When ATN is high (False) EOI goes low
(True) to indicate the end of a data transmission sequence; when ATN is low (True) and EOI is low
(True) a parallel poll of the HP-IB instruments is performed (the HP 8340B/41B does not respond
to parallel polling).
Typically, HP-IB data messages are sent as ASCII characters and are terminated with an ASCII
"LF" (line feed, decimal 10). However, when blocks of binary information are being sent LF cannot
safely be used as a terminator because the LF bit pattern could unintentionally occur in the middle
of a data sequence. To prevent a false termination, the EOI line is used to signify the true end of a
data sequence (alternatively, a byte-counting method that explicity defines the number of
expected data bytes may be used).
THE TWELVE HP-IB MESSAGES
The HP-IB control, handshake, and data lines interact to transfer information between interconnected
instruments. This information transfer process can be organized into 12 distinct categories which are,
by convention, referred to as bus messages. These messages will be explained using the following
HP-IB mnemonics:
ATN
Attention HP-IB line TRUE, indicating Command operating mode.
ATN
Attention line FALSE, indicating DATA transfer mode.
CA
Controller active state.
CR
Carriage Return ASCII decimal 13.
data
One or more ASCII data bytes (the HP 8340B/41B accepts lower-case ASCII characters,
which it automatically upshifts).
DCL
Device Clear, returns all instruments (addressed or not addressed) to an instrument-defined
state; DCL is accomplished by ASCII "DC4" (decimal 20).
GET
Group Execute Trigger, initiates a simultaneous instrument-defined response from all instru¬
ments; accomplished by ASCII "BS" (decimal 8).
GTL
Go To Local, returns instruments to local (front panel) control; accomplished by ASCII "SOH"
(decimal 1).
LA
Listener active state.
LAD
Listen Address of a specific Device (see LAG).
LAG
Listen Address Group (listen addresses of specified instruments). An HP-IB instrument may
have any unique address in the range 00-30 (decimal). The distinction between a listen
address and a talk address is made in bits 5 and 6; using address 19 as an example:
BIT
7
6
5
4
3
2
1
0
TALK
XI
0
1
0
0
1
1
LISTEN
X
0
1
10
0
1
1
The corresponding ASCII codes for the available HP-IB addresses are listed in Table 3-3.
LF
Line Feed, ASCII decimal 10.
3-86
Operating Information
HP 8340B/41B
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