RS-232↔IEEE 488 INTERFACE CONVERTER
6.4 Bus Management Lines
Five hardware lines on the GPIB are used for bus management. Signals on these lines are often referred to as uniline (single line) commands. The signals are “active low”: A low voltage represents a logical “1” (asserted), and a high voltage represents a logical “0” (unasserted).
6.4.1ATTENTION (ATN)
ATN is one of the most important lines for bus management. If Attention is asserted, then the information contained on the data lines is to be interpreted as a multiline command. If it is not, then that information is to be interpreted as data for the Active Listeners. The Active Controller is the only bus device that has control of this line.
6.4.2INTERFACE CLEAR (IFC)
The IFC line is used only by the System Controller. It is used to place all bus devices in a known state. Although device configurations vary, the IFC command usually places the devices in the Talk and Listen Idle states (neither Active Talker nor Active Listener).
6.4.3REMOTE ENABLE (REN)
When the System Controller sends the REN command, bus devices will respond to remote operation. Generally, the REN command should be issued before any bus programming is attempted. Only the System Controller has control of the Remote Enable line.
6.4.4END OR IDENTIFY (EOI)
The EOI line is used to signal the last byte of a multibyte data transfer. The device that is sending the data asserts EOI during the transfer of the last data byte. The EOI signal is not always necessary, as the end of the data may be indicated by some special character such as carriage return.
The Active Controller also uses EOI to perform a Parallel Poll by simultaneously asserting EOI and ATN.
6.4.5SERVICE REQUEST (SRQ)
When a device desires the immediate attention of the Active Controller it asserts SRQ. It is then the Controller’s responsibility to determine which device requested service. This is accomplished with a Serial Poll or a Parallel Poll.
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