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National Instruments 779651-01 High-Speed Bus Isolator/Expander 

National Instruments part number 779651-01 is an identifying part number for the GPIB-120B, a High-Speed Bus Isolator/Expander. This model is fully compatible with IEEE 488.1 and IEEE 488.2 standards, allowing the user to expand the limit of allowable devices on the GPIB bus to as many as 28. This makes the Expander/Isolator suitable for a variety of test and measurement systems. The 779651-01 Bus Isolator Expander offers excellent isolation capabilities, providing up to 2,500 VDC of non-continuous isolation between the GPIB ports, the power supply, and each individual GPIB port. 

With this model, bidirectional expansion of GPIB system devices is possible, enabling talkers, listeners, and controllers to operate on either side of the expansion. Functionally, this device operates transparently, allowing GPIB control and communication programs that work with the unexpanded system to be used seamlessly with the expanded system. Both buffered and unbuffered data transfer modes are available with the 779651-01 Expander/Isolator. By default, the device is set to unbuffered transfer mode. However, users can easily select their preferred data transfer mode using the switch located on the back of the unit. 

The NI 779651-01 serves as an isolating interface between instruments and an IEEE 488 bus controller, effectively reducing induced common-mode noise and ground loop issues. Adding one 779651-01 increases the maximum device limit on the GPIB bus from 15 to 28 devices. Additionally, it extends the cable length limit for the system by an additional 4 meters, totaling up to 20 meters, depending on the number of devices connected on that side of the expansion.It operates transparently, meaning existing GPIB control and communication programs can seamlessly work with an expanded system without modification.