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Chapter 7 GPIB Programming Techniques
© National Instruments Corp. 7-3 NI-488.2 User Manual for Windows
cable in your system. It is called the Configure (CFGn) message. Because HS488 can
operate only with cable lengths of 1 to 15 meters, only CFGn values of 1 through 15 (hex
61 through 6F) are valid. If the cable length was configured correctly in wibconf, you
can determine how many meters of cable are in your system by calling ibask (option
IbaHSCableLength) in your application program. For CFE and CFGn messages,
refer to Appendix A, Multiline Interface Messages, in the NI-488.2 Function Reference
Manual for DOS/Windows.
System Configuration Effects on HS488
Maximum data transfer rates can be limited by your host computer and GPIB
system setup. For example, even though the theoretical maximum transfer rate
with HS488 is 8 Mbytes/s, the maximum transfer rate obtainable on PC compatible
computers with an ISA bus is 2 Mbytes/s. The same IEEE 488 cabling constraints
for a 350 ns T1 delay apply to HS488. As you increase the amount of cable in
your GPIB configuration, the maximum data transfer rate using HS488 decreases.
For example, two HS488 devices connected by two meters of cable can transfer
data faster than three HS488 devices connected by four meters of cable.
Waiting for GPIB Conditions
You can use the ibwait function to obtain the current ibsta value or to suspend your
application until a specified condition occurs on the GPIB. If you use ibwait with a
parameter of zero, it immediately updates ibsta and returns. If you want to use
ibwait to wait for one or more events to occur, then pass a wait mask to the function.
The wait mask should always include the TIMO event; otherwise, your application is
suspended indefinitely until one of the wait mask events occurs.
Device-Level Calls and Bus Management
The NI-488 device-level calls are designed to perform all of the GPIB management for
your application program. However, the NI-488.2 driver can handle bus management
only when the GPIB interface board is CIC (Controller-In-Charge). Only the CIC is able
to send command bytes to the devices on the bus to perform device addressing or other
bus management activities. Use one of the following methods to make your GPIB board
the CIC:
• If your GPIB board is configured as the System Controller (default), it automatically
makes itself the CIC by asserting the IFC line the first time you make a device-level
call.