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Appendix B: GPIB System Concepts
2714 & 2715 Programmer Manual
B–13
Note that one handshake cycle transfers one data byte. The listeners must then
reset the NRFD line high and the NDAC line low before the talker asserts DAV
for the next data byte transfer. An invalid state on the bus is indicated when
NRFD and NDAC are high at the same time.
The management bus is a group of five signal lines that are used to control the
operation of the IEEE Std 488.1 (GPIB) Digital Interface.
The system controller is the only instrument on the bus allowed to assert IFC.
IFC is asserted for greater-than 100
m
s to place all instruments in a predeter-
mined state. While IFC is being sent, only the DCL (Device Clear), LLO (Local
Lockout), PPU (Parallel Poll Unconfigure), and REN (Remote Enable) interface
messages (universal commands) will be recognized.
The controller in charge is the only instrument on the bus allowed to assert ATN.
ATN is asserted when an instrument connected to the bus is being enabled as a
talker or listener, or when sending other interface control messages. As long as
the ATN line is asserted (low), only instrument address codes and interface
control messages are sent over the bus. When the ATN line is unasserted, only
those instruments enabled as a talker and listener can send and receive data over
the bus.
Any instrument connected to the bus can request the controller’s attention by
asserting the SRQ line. The controller responds by asserting ATN and executing
a serial poll routine to determine which instrument is requesting service. The
instrument requesting service responds with a device-dependent status byte with
bit seven asserted.
When the instrument requesting service is found, program control is transferred
to a service routine for that instrument. When the service routine is completed,
program control returns to the main program. The controller does not have to see
the SRQ line asserted to perform a polling routine; it may do so whenever a
program requires it.
The system controller asserts the REN signal line whenever the interface system
operates under remote program control. The REN signal causes an instrument on
the bus to select between two alternate sources of programming data. It is used
with other interface control messages such as LLO (Local Lockout) or GTL (Go
To Local). A remote-local interface function indicates to an instrument that the
instrument will use either information input from the interface (remote) or
information input by the operator via the front panel controls (local).
Management Bus
IFC (Interface Clear)
ATN (Attention)
SRQ (Service Request)
REN (Remote Enable)
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