NI-488.2M Software Description
Chapter 1
NI-488.2M SRM for OS/2
1-6
© National Instruments Corp.
Controller-In-Charge and System Controller
You can have multiple Controllers on the GPIB, but only one Controller at a
time can be the active Controller, or Controller-In-Charge (CIC). When a
Controller is not active, it is considered an idle Controller. Active control can
pass from the current CIC to an idle Controller. The System Controller, usually
a GPIB interface board, is the only device on the bus that can make itself the
CIC.
Sending Messages Across the GPIB
Devices on the bus communicate by sending messages. Signals and lines
transfer these messages across the GPIB interface, which consists of 16 signal
lines and 8 ground return (shield drain) lines. The 16 signal lines are discussed
in the following sections.
Data Lines
Eight data lines, DIO1 through DIO8, carry both data and command messages.
Handshake Lines
Three hardware handshake lines asynchronously control the transfer of message
bytes between devices. This process is a three-wire interlocked handshake, and
it guarantees that devices send and receive message bytes on the data lines
without transmission error. Table 1-1 summarizes the GPIB handshake lines.
Table 1-1. GPIB Handshake Lines
Line
Description
NRFD (not ready for data)
Listening device is ready/not ready to
receive a message byte.
NDAC (not data accepted)
Listening device has/has not accepted a
message byte.
DAV (data valid)
Talking device indicates signals on data
lines are stable (valid) data.
