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Theory of Operation
Chapter 3
The circuitry on each side of the expansion monitors local GPIB states and converts the signals
monitored into X signals (for transmit) which drive the R signals (for receive) on the opposite
side of the expansion. The X signals are optically isolated from the R signals. Each GPIB signal
is sensed or driven depending on the System Controller, Active Controller, and Source
Handshake states of each side of the expansion.
The GPIB-120A Front Panel
The GPIB-120A front panel has seven light-emitting diodes (LEDs). The POWER LED on the
left-hand side of the front panel is lit whenever you power on the GPIB-120A.
For each bus, an LED indicates the status of the System Controller, Active Controller, or Source
Handshake state.
Power On
When you power on the GPIB-120A, all circuitry is cleared to an initialized state. The
GPIB-120A expansion system is fully operational when you power on the GPIB-120A and your
instruments are connected. Where there is GPIB activity, it is recommended that you keep at
least two-thirds of the devices on both buses powered on.
System Controller Detection
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
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.
If a GPIB device on Bus B asserts ATN, the Bus B Active Controller state becomes true and the
Bus A Active Controller state becomes false.
Source Handshake Detection
A device is considered a source handshaker if it is an active Controller sourcing command bytes
or if it is a Talker sourcing data bytes.
After you power on, Bus A and Bus B Source Handshake states are false.
GPIB-120A User Manual
3-2
© National Instruments Corporation
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