National Instruments GPIB-120A 用户手册

                                                                                                                                                                                              

This chapter describes the operational theory of the GPIB-120A.  This chapter assumes that you
have a basic knowledge of the GPIB.  If you are a first-time user or you would like to review the
basics, refer to Appendix A, Operation of the GPIB, for a history and the basic operation of the
GPIB.

The GPIB-120A consists of two sides:  Bus A and Bus B.  The circuitry for both sides is
logically identical and the two sides are electrically isolated from each other.  The only
difference between the two sides is that logic ground on Bus A is connected to the chassis
ground while the logic ground for Bus B is not.  Thus, Bus B is the isolated side of the
expansion.  To reduce measurement problems caused by noise and ground loops, the
measurement instruments must be located on the isolated Bus B and all other devices must be
located on Bus A.  A block diagram for the GPIB-120A is shown in Figure 3-1.

GPIB

Transceivers

Optoisolators

Controller

Detection

GPIB A

GPIB B

GPIB

Transceivers

Controller

Detection

Side B

Side A

Source

Handshake

Detection

Source

Handshake

Detection

Data

Direction

Control

Data

Direction

Control

Parallel

Poll

Detection

Parallel

Poll

Detection

Figure 3-1.  GPIB-120A Block Diagram