© National Instruments Corporation
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GPIB-120A User Manual
Chapter 2
Connection
This chapter contains information for connecting your GPIB-120A.
Danger:
The GPIB-120A does not generate high voltages; however, you must use extreme
caution if your system or application can cause high voltages on the shield or logic
ground of the GPIB cable you attach to the electrically isolated connector (GPIB
Port B) located on the rear panel of the GPIB-120A. Many GPIB cables use a
metal shell to enclose the GPIB piggyback connectors. This metal shell is normally
connected to the cable shield. Therefore, if the shield is at a high potential with
respect to the chassis ground of the GPIB-120A, the high voltage is also present, in
most cases, on the cable connector housing.
The chassis, back panel, and front panel of the GPIB-120A are electrically
connected to the earth ground pin of the AC power connector on the back panel. If
the AC wiring in your building is installed correctly, you can safely operate the
GPIB-120A. Because the EMI shield and jack screws of rear panel connector
‘GPIB A’ are electrically shorted to the rear panel, a GPIB cable attached to GPIB
Port A has its shield and connector housing connected to earth ground, as well,
and no danger should exist associated with GPIB Port A. There may be, however,
a hazardous voltage potential between the cable connector housings of the GPIB
cable attached to GPIB Port A and the GPIB cable attached to GPIB Port B.
Never touch the GPIB-120A case, power cord, or any GPIB cable connected to the
GPIB-120A unless you are absolutely sure that there are no hazardous voltages
present. In addition, never open the GPIB-120A case unless you have removed the
power cord and all GPIB cables from the back panel.
The GPIB-120A is not recommended for use when ground potential differences in
excess of 30 V are present between the two rear panel GPIB connectors and
associated cables unless sufficient precautions are taken to ensure human safety.
Isolation Considerations
Noise and ground loop problems are common when performing small signal analysis. You can
solve such problems by isolating a group of sensitive GPIB instruments from other noisier
instruments.
Your GPIB-120A has two GPIB connectors that are electrically isolated from each other: Bus A
and Bus B. Bus B is also isolated from the chassis and back panel of the GPIB-120A.
