National Instruments NI-488.2 System Configuration Effects on HS488

Chapter 7 NI-488.2 Programming Techniques

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 NI-488.2 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 NI-488.2

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 a PC-compatible computer with ISA bus, the

maximum transfer rate obtainable is only 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 four HS488

devices connected by 4m of cable.