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Important GPIB Features
The important GPIB features are described below. For detailed information about GPIB functionality, refer to the appropriate references in Appendix A, "Selected Bibliography."
The Bus and Connector
The GPIB bus is a cable with two 24-pin connectors that allow you to connect multiple devices to each other. The bus and connector have these features and limitations:
- You can connect up to fifteen devices to a bus.
- You can connect devices in a star configuration, a linear configuration, or a combination of configurations.
- To achieve maximum data transfer rates, the cable length should not exceed 20 meters total or an average of 2 meters per device. You can eliminate these restrictions by using a bus extender.
GPIB Devices
Each GPIB device must be some combination of a Talker, a Listener, or a Controller. A Controller is typically a board that you install in your computer. Talkers and Listeners are typically instruments such as oscilloscopes, function generators, multimeters, and so on. Most modern instruments are both Talkers and Listeners.
- Talkers -- A Talker transmits data over the interface when addressed to talk by the Controller. There can be only one Talker at a given time.
- Listeners -- A Listener receives data over the interface when addressed to listen by the Controller. There can be up to 14 Listeners at a given time. Typically, the Controller is a Talker while one or more instruments on the GPIB are Listeners.
- Controllers -- The Controller specifies which devices are Talkers or Listeners. A GPIB system can contain multiple Controllers -- one of which is designated the System Controller. However, only one Controller can be active at a given time. The current active controller is the Controller-In-Charge (CIC). The CIC can pass control to an idle Controller, but only the System Controller can make itself the CIC.
- When the Controller is not sending messages, then a Talker can send messages. Typically, the CIC is a Listener while another device is enabled as a Talker.
Each Controller is identified by a unique board index number. Each Talker/Listener is identified by a unique primary address ranging from 0 to 30, and by an optional secondary address, which can be 0 or can range from 96 to 126.
GPIB Data
There are two types of data that can be transferred over the GPIB: instrument data and interface messages.
- Instrument data -- Instrument data consists of vendor-specific commands that configure your instrument, return measurement results, and so on. For a complete list of commands supported by your instrument, refer to its documentation.
- Interface messages -- Interface messages are defined by the GPIB standard and consist of commands that clear the GPIB bus, address devices, return self-test results, and so on.
Data transfer consists of one byte (8 bits) sent in parallel.The data transfer rate across the interface is limited to 1 megabyte per second. However, this data rate is usually not achieved in practice, and is limited by the slowest device on the bus.
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