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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).
6.4.1 A
TTENTION
(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.
6.4.2 I
NTERFACE
C
LEAR
(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).
6.4.3 R
EMOTE
E
NABLE
(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.
6.4.4 E
ND
O
R
I
DENTIFY
(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.
6.4.5 S
ERVICE
R
EQUEST
(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.