Measurement Computing Personal488 rev.3.0 - Manual (Page 139)

Personal488 for Windows 95/98/Me/NT/2000 03/16/01 Troubleshooting C-1
Appendix C
The IEEE 488 Bus Standard …… C-1
Analyzing the IEEE Bus …… C-2
Common Problems and Solutions …… C-3
New Standards Simplify Programming …… C-6
Frequently Asked Personal 488 Questions …… C-7
Troubleshooting IEEE 488 Systems and Software
To efficiently diagnose, troubleshoot and verify IEEE 488 systems, you should first have some
basic knowledge of the IEEE bus. Since the hardware portion of the IEEE standard is rigorous
and stable, most of the problems you will encounter during the system integration process will be
in the application software. This note contains a brief IEEE tutorial followed by troubleshooting
techniques.
The IEEE 488 Bus Standard
Addresses
Each device on the IEEE 488 bus has a unique address — even the controller. The addresses
range from 0 to 30. Addresses are the means by which controllers select specific instruments. To
send data to a device, the controller must both address itself to talk and address the device to
listen.
Commands
IEEE 488 recognizes two types of commands: Device-Dependent Commands (DDCs) and IEEE
488-specific commands. DDC's such as “R0F0T2X” are simply data sent from one device to
another (in this instance from the controller to an instrument). IEEE 488-specific commands
come in two forms; multiline commands and uniline commands. Multiline commands are sent on
the data bus lines; uniline commands are individual signals on the bus.
The uniline signals are the easiest to decipher because each signal has one specific purpose. All
but two of these signals are issued exclusively by the controller.
Uniline Commands
• Interface Clear (IFC) is the most dramatic uniline command. It stops all activities on the
bus and returns the interface of every device to a quiescent state.
• Remote Enable (REN) informs the devices on the bus that the IEEE interface is active. It
does not lock out an instrument’s front panel.
• Attention (ATN) is the signal that differentiates data from multiline commands on the
data bus. When ATN is asserted by the controller, the bits on the data bus are actually
multiline commands being issued to all of the devices on the bus.
• End Or Identify (EOI) can be issued by any device talking on the bus. Talkers use EOI to
notify listeners that the end of the transmission has taken place.
• Service Request (SRQ) can be issued by any device on the bus. It allows devices on the
bus to interrupt, or alert, the controller to an internal situation that needs servicing (e.g.
“my buffer is full,” “I’ve encountered an error,” “my trigger has been satisfied”).