n
|
|
GPIB Operation 3
I/O Buffers
IEEE 488.1
STANDARD MESSAGES
Device Clear
REN (Remote ENable): The controller uses this line to place de-
vices in remote or local program mode.
SRQ (Service ReQuest): Any device can drive the SRQ line true
to asynchronously request service from the controller. This is the
equivalent of a single interrupt line on a computer bus.
EOI (End Or Identify): This line has two purposes. The talker
uses it to mark the end of a message string. The controller uses it to
tell devices to identify their response in a parallel poll (discussed
later in this section).
The instrument has a 256-byte input buffer and a 256-byte output
buffer. An incoming program message is not decoded before a
message terminator has been received. However, if the input buff-
er becomes full (because the program message is longer than the
buffer), the instrument starts analyzing the message. In this case
data transmission is temporarily halted, and the controller may
generate a timeout if the limit was set too low.
The IEEE 488.1 standard specifies not only the mechanical and
electrical aspects of the GPIB, but also the low-level transfer pro-
tocol, e.g. it defines how a controller addresses devices, turns
them into talkers or listeners, resets them or puts them in the re-
mote state. Such interface messages are executed with the
interface management lines of the GPIB, usually with ATN true.
All of these messages (except GET) are executed immediately
upon reception and not in chronological order with normal com-
mands.
Note: In addition to the IEEE 488.1 interface message standards,
the IEEE 488.2 standard specifies some standardized program
messages, i.e. command headers. They are identified with a lead-
ing asterisk <*> and are listed among the commands in Section 5.
The command list in Section 5 does not contain any command for
clearing the input/output buffers or for setting the instrument to
the remote state. This is because such commands are already spe-
cified as IEEE 488.1 standard messages. Refer to the GPIB
interface manual of the host controller as well as to its support
programs which should contain special calls for the execution of
these messages.
The following describes those IEEE 488.1 standard messages
which go beyond mere reconfiguration of the bus and which have
an effect on the operation of the instrument.
In response to a universal Device CLear (DCL) or a Selected De-
vice Clear message (SDC), the oscilloscope clears the input/output
13
To Next Page
Loading...
Need help?
General
