Device Function Calls - National Instruments GPIB-PC User Manual

GPIB-PC Functions — Overview Section Four

Device Function Calls

Device functions are those functions in which the unit descriptor
identifies a device rather than an interface board. There are some
activities common to all device functions that should be understood
thoroughly.
In a single board configuration in which there is only one GPIB-PC in
use, when the first device function of the program is executed, the GPIB
is initialized by its controlling access board with the Interface Clear
(IFC) command. The Remote Enable (REN) line on that GPIB is also
asserted. If selected in the configuration program, the Local Lockout
(LLO) command is also sent to all devices on the GPIB to place them
in a lockout state. Furthermore, the device may be addressed to listen
and then unaddressed before certain functions are executed. This is to
ensure that the device is in remote program mode.
In a multiboard configuration in which there is more than one GPIB-PC,
the process is the same as previously described, with the exception that
each GPIB is initialized by its access board when the first device on
that GPIB is accessed by a device function call.
The previous descriptions assume that the GPIB-PC is the System
Controller of its GPIB, which is the usual configuration. If the GPIB-PC
is not the System Controller, it must be passed CIC authority from the
System Controller to execute device functions. If the access board is
not CIC when a device function is called, the board requests service
from the current CIC by asserting the Service Request (SRQ) line and
passing the status response byte hex 41 when serially polled by the CIC.
The handler then waits indefinitely for control to be passed to the access
board. The computer system hangs if there is not another CIC that will
pass control. This might happen, for example, if the access board is
supposed to be System Controller, but it was not configured as such
during software installation.
In Revision D software, if the access board is not CIC when the device
call is made, the ECIC error is returned.
Also in Revision D software, because of multitasking, board-level
reconfiguration functions must not be called during device-level calls.
GPIB-PC User Manual 4-12 ©National Instruments Corp.