Technically Speaking
IEEE 488.
IEEE 488 (also known as GPIB or General Purpose Interface Bus) is an international standard for a parallel interface that has greatly simplified the connection of sensors and programmable instruments to a computer. With it, instru- ments from different manufacturers can be connected by
a single standard cable.
Two IEEE 488 standards are in use: the older IEEE 488.1 standard, which deals with the hardware only, and the newer IEEE 488.2 standard, which also addresses software issues like data formats and error handling.
IEEE 488.1 is a clearly defined mechanical, hardware, and electrical protocol specification. It doesn’t address data formats, status reporting,
IEEE 488.2 enhances the IEEE 488.1 standard by specifying data formats, status reporting, error handling, controller functionality, and common instrument commands. It focuses mainly on the software protocol issues and thus maintains compatibility with the
Most devices can be adapted to the IEEE 488 specification. The specification says nothing about the function of the device itself, or about the form of the device’s data. Instead, it defines a separate interface that can be added to the device. Only the signals passing into the interface from the IEEE 488 bus and from the device are defined in the standard.
There are three classes of devices that can be connected to the IEEE 488 bus: Listeners, Talkers, and Controllers. Some devices include more than one of these functions. The IEEE 488 standard allows a maximum of 15 devices to be connected on one bus. A minimum system consists of one Controller and one Talker or Listener device.
A Controller is the device that sends instructions. It’s
possible to have several Controllers on the bus at once but only one may be active at a time. The Controller that’s in charge at the moment is called the Active Controller.
The Controller that’s in charge of the entire bus is called the System Controller. It has several unique capabilities, including the ability to send Interface Clear (IFC) and Remote Enable (REN) commands. IFC clears all device interfaces and returns control to the System Controller. REN allows devices to respond to bus data once they are addressed to listen. The System Controller may optionally pass control to another Controller, which then becomes the Active Controller.
A Listener is a device that can receive data from the bus when instructed by the Controller. A Talker transmits data on the bus when instructed. The Controller can set up a Talker and a group of Listeners to send data between groups of devices.
The IEEE 488 interface system consists of 16 signal lines and 8 ground lines. The 16 signal lines are divided into 3 groups (8 data lines, 3 handshake lines, and 5 interface- management lines).
The lines DIO1 through DIO8 are used to transfer addresses and control information and data. The formats for addresses and control bytes are defined by the IEEE 488 standard. Data formats are undefined and may be ASCII or binary. DIO1 is the Least Significant Bit.
The three handshake lines (NRFD, NDAC, DAV) control the transfer of message bytes among devices and form the method for acknowledging the transfer of data. This handshaking process guarantees that bytes on the data lines are sent and received without any transmission errors. It’s one of the unique features of the IEEE 488 bus.
The Not Ready for Data (NRFD) handshake line is asserted by a Listener to indicate it is not yet ready for the next data or control byte. Note that the Controller will not see NRFD released (meaning the devices are ready for data) until all devices have released it.
The Not Data Accepted (NDAC) handshake line is asserted by a Listener to indicate it has not yet accepted the data or control byte on the data lines. Note that the Controller will not see NDAC released (i.e., data accepted) until all devices have released it.
The Data Valid (DAV) handshake line is asserted by the Talker to indicate that a data or control byte has been placed on the data lines and has had the minimum specified stabilizing time. The byte can now be safely accepted by the devices.
Five interface management lines (ATN, EOI, IFC, REN, SRQ) manage the flow of control.
The Attention (ATN) signal is asserted by the Controller to indicate that it is placing an address or control byte on the data bus.
The End or Identify (EOI) signal has two uses. A Talker may assert EOI simultaneously with the last byte of data to indicate
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