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Microlink Measurement and Control Systems

Technical Notes: GPIB


The GPIB (general purpose interface bus) was specifically designed to connect computers, peripherals and laboratory instruments so that data and control information could pass between them. It is also known as IEEE-488 or HPIB, and is electrically equivalent to IEC-625 bus. It is defined completely in the IEEE standard 488.1-1987 Standard Digital Interface for Programmable Instrumentation.

To use the GPIB you need a GPIB adaptor card in your computer and a GPIB lead. Fourteen devices can be connected to one GPIB and data can be transferred at up to 200000 bytes per second. However, devices shouldn't be more than a couple of metres from the computer.

A GPIB adaptor from Biodata comes with driver software and interfaces to popular programming languages. Biodata also supply ready-to-run applications for data acquisition over the GPIB, where no programming is required and no understanding of the GPIB is necessary.

Technically, the GPIB uses a 16 line parallel connection which has strictly defined mechanical and electrical properties. The 16 lines are divided into 8 data lines, 3 handshake lines to synchronise the transfer and 5 management lines to control use of the bus.

At any time there must be one device on the bus which is the controller. This device issues commands to other devices, and in our systems is always the computer. Other devices may be Talkers - putting data onto the bus, Listeners - reading from the bus, or inactive - neither talking nor listening. Only 1 device may talk at once, but more than 1 may listen to the Talker.

Microlink and The GPIB

The Microlink 3000 system can use GPIB communications. The Microlink frame is addressed as a single device on the GPIB, which means that up to 14 frames could be used. Individual modules within a frame are accessed through the use of secondary addresses, also referred to as extended GPIB addresses.

The Microlink recognises and uses the following features of the GPIB.

Extended Listen Address
A two byte address which selects a module to listen on the GPIB, so it will receive data from a talker on the bus.
Unlisten
A command to unaddress a frame which has been addressed as a listener
Extended Talk Address
A two byte address which selects a module to talk on the GPIB, so it will be able to send out data on the bus
Untalk
A command to unaddress a frame which has been addressed as a talker
Interface Clear (IFC)
A pulse on the IFC line of the GPIB which returns the frame and all modules to the power-up state.
Service Request (SRQ)
Some modules can set the SRQ line of the bus true to indicate that a particular event has occurred.
End Or Identify (EOI)
Some modules set the EOI line true when they send the last byte of their data to indicate the end of the message.

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