© National Instruments
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2
Hardware Overview
This chapter describes your GPIB isolator/expander.
GPIB-120B LEDs
The GPIB-120B has seven light-emitting diodes (LEDs). The POWER LED on the left side of
the isolator/expander is lit whenever you power on the GPIB-120B.
For each bus, an LED indicates the status of the System Controller, Active Controller, or Source
Handshake state, as shown in Figure 2-1. LEDs associated with Bus A are green, while those
associated with Bus B are amber.
Figure 2-1.
Front View GPIB-120B
Power On (PWR)
When you power on the GPIB-120B, all circuitry is cleared to an initialized state. The
isolation/expansion system is fully operational when you power on the GPIB-120B and your
instruments are connected. Where there is GPIB activity, keep at least two-thirds of the devices
on both buses powered on.
System Controller Detection (SC)
After you power on, Bus A and Bus B System Controller states are false. If a GPIB device on
Bus A asserts IFC or REN, the Bus A System Controller state becomes true, and the Bus B
System Controller state becomes false.
If a GPIB device on Bus B asserts IFC or REN, the Bus B System Controller state becomes true
and the Bus A System Controller state becomes false.
Active Controller Detection (AC)
After you power on, Bus A and Bus B Active Controller states are false. If a GPIB device on
Bus A asserts ATN, the Bus A Active Controller state becomes true and the Bus B Active
Controller state becomes false.
I
0
GPIB-120B
Isolator / Expander
PWR
SC
AC
SH
SC
AC
SH
