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Triggerable general-purpose interface bus controller
1.Throughout this article we will also use the word “bus” to refer to the GPIB.
2.S. F. Owen and D. S. Hall, Rev. Sci. Instrum. 75, 259 (2004).
3.Software for the GPIB computer, including several other examples and other supporting information and materials, can be obtained by request from the author.
4.A compendium of information about the parallel port may be found at [http://www.lvr.com/parport.htm], for example.
5.NI-488.2 User Manual for DOS (National Instruments, Austin, TX, 1996). The discussion on pp. 2–20ff. is most relevant to programming a computer to emulate a GPIB device on the bus.
6.The process of transferring bus control from one computer to another is surprisingly simple (see Ref. 8). The computer that has control passes it by invoking the ibpct (“pass control”) command. The computer receiving control simply waits until the controller-in-charge bit (bit 5) is set in its status word, at which point it is the bus controller. To pass control back, the roles of the two computers are reversed. The main computer, as system controller, may always wrest control of the bus back from the GPIB computer by sending the commands ibrsc 1 (“request system control”) and ibsic (“send interface clear”) in sequence.
7.This step occurs after control is passed to the GPIB computer so that it can determine the addresses of the devices on the bus. There may be a delay while the commands are queued if the sequence is long, and it is important not to begin triggering until the sequence is completely queued for output. We found that the BUSY signal can be used to indicate that the queueing process is complete.
8.NI-488.2 Function Reference Manual for DOS/Windows (National Instruments, Austin, TX, 1996). This reference contains the 488.2 command set, including the commands that pass control back and forth between two GPIB controllers.
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