National Instruments GPIB-120A The GPIB-120A Front Panel

Theory of Operation Chapter 3

GPIB-120A User Manual 3-2 © National Instruments Corporation

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.

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.

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.

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.

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.