ICS ELECTRONICS 4863 Manual page 135

system to 15). Only one controller can be the controller-in-charge at
any given time. Control originates with the system controller and is
passed back to other controller(s) as required. Control can be passes
back to the system controller or to another controller after the
completion of the task. The system controller has the capability of
taking control back at any time and resetting all addressed devices to
their unaddressed state.
Each bus device is identified by a five-bit binary address. There are
31 possible primary addresses 0 through 30. Address 31 is reserved
as the 'untalk' or 'unlisten' command. Some devices contain
subfunctions, or the 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
A
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
A-4