A-4
A
devices themselves may be addressed by a secondary five-bit binary
address immediately following the primary address, i.e. 1703. This
secondary address capability expands the bus address range to 961 addresses.
Most bus addresses are set at the time the system is configured by rocker
switches which are typically located on each devices' rear panel. Devices
that are SCPI 1991 compatible, can have their bus address set by a GPIB
SYSTEM configuration command.
Information is transmitted on the data lines under sequential control of the
three handshake lines. No step in the sequence can be initiated until the
previous step is completed. Information transfer proceeds as fast as the
devices respond (up to 1 Mbs), but no faster than that allowed by the slowest
addressed device. This permits several devices to receive the same message
byte at the same time. Although several devices can be addressed to listen
simultaneously, only one device at a time can be addresses as a talker. When
a talk address is put on the data lines, all other talkers are normally
unaddressed.
ATN (attention) is one of the five control lines and is set true by the
controller-in-charge while it is sending interface messages or device
addresses. The messages are transmitted on the seven least significant data
lines and are listed in the MSG columns in Table A-1. When a device is
addressed as a talker, it is allowed to send device-dependent messages (e.g.,
data) when the controller-in-charge sets the ATN line false. The data
messages are typically a series of ASCII characters ending in a CR, LF, or
CR LF sequence. The data messages often consist of eight-bit binary
characters and end on a predetermined count or when the talker asserts the
EOI line simultaneously with the last data byte. The controller-in-charge
must be programmed to correctly respond to each device's message
termination sequence to avoid hanging-up the system or leaving characters
that will be output when the device is addressed as a talker again.
IFC (interface clear) is sent by the system controller and places the interface
system in a known quiescent state with all devices unaddressed.
REN (remote enable) is sent by the system controller and is used with other
interface messages or device addresses to select either local or remote
control of each device.
SRQ (service request) is sent by any device on the bus that wants service,
such as counter that has just completed a time-interval measurement.
