HP 3468A Operator's Manual page 103

The following paragraphs define the terms and concepts used to describe HP-IL
system operations.
a. Address: Each device in the loop is assigned an address by the controller. The
address is used to specify which device in the loop will receive information or
send information.
b. Byte: A byte is a unit of information consisting of 8 binary digits called bits.
c. Device: Any instrument or unit that is HP-IL compatible is called a device.
d. Device Dependent: An action a device performs in response to information
sent through the loop. The action is characteristic of a particular instrument
and will probably vary from device to device.
e. Frame: Messages are sent through the loop as a sequence of eleven bits call-
ed a message "frame".
f. Polling: Polling is a process typically used by a controller to locate a device
that has requested service from the controller. There are two types of polling,
Serial Poll and Parallel Poll:
1. Serial Poll. When the controller executes a serial poll, the addressed device
sends one byte of operational information called a status byte. If more
than one device in the loop is capable of requesting service, each device
in the loop must be serial polled until the device that requested service is
located.
2. Parallel Poll. This method obtains a status bit from eight devices in the
loop. The 3468A does not respond to a parallel poll.
Messages
Every message sent through the loop is sent as a sequence of eleven bits called a
message frame. Commands, such as Listen, are made up of one or more message
frames. The first bit in each message frame is a sync bit and is specially coded so
that each device can recognize the beginning of a frame. The sync bit and the two
following bits are called control bits and are used to determine the classification of
the message frame. There are three major classifications of frames: Command,
Ready, and Data. The remaining eight bits are the data bits and specify the par-
ticular message within the classification.
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