National Instruments NI-488.2 - Manual (Page 75)

Chapter 8 NI-488.2 Programming Techniques
© National Instruments Corporation 8-3 NI-488.2 User Manual
If you are using device-level calls, the NI-488.2 software automatically
sends the HS488 configuration message to devices. If you enabled the
HS488 protocol in the GPIB Configuration utility, the NI-488.2 software
sends out the HS488 configuration message when you use
ibdev to bring
a device online. If you call ibconfig to change the GPIB cable length, the
NI-488.2 software sends out the HS488 message again, the next time you
call a device-level function.
If you are using board-level traditional NI-488.2 calls or multi-device
NI-488.2 calls and you want to configure devices for high-speed, you
must send the HS488 configuration messages using
ibcmd or SendCmds .
The HS488 configuration message is made up of two GPIB command
bytes. The first byte, the Configure Enable (CFE) message (hex 1F),
places all HS488 devices into their configuration mode. Non-HS488
devices should ignore this message. The second byte is a GPIB secondary
command that indicates the number of meters of cable in your system.
It is called the Configure (CFGn) message. Because HS488 can operate
only with cable lengths of 1 to 15 m, only CFGn values of 1 through 15
(hex 61 through 6F) are valid. If the cable length was configured properly
in the GPIB Configuration utility, you can determine how many meters of
cable are in your system by calling
ibask (option IbaHSCableLength )
in your application. For more information about CFE and CFGn messages,
refer to the Multiline Interface Messages topic in the NI-488.2 online help.
For instructions on accessing the online help, refer to the
Using the
NI-488.2 Documentation section in About This Manual .
System Configuration Effects on HS488
Maximum HS488 data transfer rates can be limited by your host computer
and GPIB system setup. For example, when using a PC-compatible
computer with PCI bus, the maximum obtainable transfer rate is
8 Mbytes/s, but when using another bus, such as USB or Ethernet,
the maximum data transfer rate depends on the maximum transfer rate
of that bus.
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 four HS488 devices connected by 4 m of cable.