Measurement Computing Personal488 rev.3.0 - Manual (Page 140)

C-2 Troubleshooting 03/16/01 Personal488 for Windows 95/98/Me/NT/2000
Multiline Commands
Now that you have seen the uniline commands and have a sense for their application, we’ll
discuss multiline commands. Remember, multiline commands are transmitted from the controller
to the devices on the data bus. The devices know that they are not data because the ATN line is
asserted. When ATN is asserted by the controller all of the devices must listen to the commands.
Multiline commands serve several functions, most notably to address instruments to talk or listen.
To address a device to listen, the controller will assert ATN and place the listen address of the
selected device on the data bus. There are 31 listen addresses. These are called the Listen Address
Group (LAG). A similar process is used for the Talk Address Group (TAG).
Most IEEE drivers, including IOtech’s Driver488 for our line of IEEE controllers, have high
level commands that perform several elemental IEEE 488 operations. In the IOtech Driver488
manual, every command explanation contains a field called BUS STATES. In BUS STATES, a
complete explanation of what is happening on the bus is displayed. For example, let’s examine
IOtech’s Driver488 command ENTER, which simply gets one reading from a specified device:
BUS STATES:
ATN•UNL, MLA, TAG, *ATN, data..., ATN
First, this indicates that ATN is asserted. Next, the multiline command UNListen (UNL) instructs
all devices that were in the listen state to exit that state. The controller then issues My Listen
Address (MLA), its own address in the listen address group, and issues the Talk Address Group
(TAG) for the specified device. Next, it unasserts ATN, which notifies the addressed device that
it may now transmit its data. Finally, after the data has been sent (perhaps ending with an EOI),
the controller once again asserts ATN.
Analyzing the IEEE Bus
The simplest way to decipher the controller’s operations and the response of the instruments,
regardless of what software or hardware you are using, is with an IEEE analyzer. Analyzer488
from IOtech allows the programmer to view all of the transactions on the bus in real time or to
record them into its 32K non-volatile transaction buffer for later inspection.
The following example problems are all diagnosed using the Analyzer488. Analyzer488 can be
operated as a portable bench-top analyzer from its easy to use keypad, or from the included
Analyst488 PC software. Analyzer488 allows the events on the IEEE bus to be monitored, stored
and analyzed. It can also be used to control devices on the bus for exercising and verifying
instrument operation. The Analyzer488 will automatically translate the state of the data bus and
control lines into easy to read IEEE messages or ASCII equivalents like SPE, TAG16, CR, and
LF. Along with its large capture buffer, Analyzer488 contains a comprehensive set of trigger
features that allow the desired group of transactions to be easily pinpointed and identified.