Page 1065 - The Mechatronics Handbook

Controllers, Talkers, and Listeners

The controller carries out the general management of the bus. While there can be many controllers
connected to the GPIB network, there can be only one controller-in-charge (CIC) which manages the
bus at that given time. All information sent out by the controller on the data lines are called “commands”
and all information sent out by other devices are termed “data.” The GPIB devices that send data any
time are called “talkers” and the devices that receive data are called “listeners.” While there can be more
than one listener operating at any given time, there can be only one talker operating at any given time.
A system can have permanent talkers and permanent listeners; however, if the capability exists, a GPIB
device can be a listener at one time and a talker at another time. A brief explanation of the signal lines
is given below, as it would enhance our understanding of the operation of GPIB interface.

Interface Management Lines
Attention (ATN)
The ATN line is controlled by the CIC. When asserted, the signals on the data lines constitute a command
signal and all devices must listen. When unasserted, the signals on the data lines represent data and are
generally sent by a talker to one or more listeners.
Interface Clear (IFC)
The IFC line is asserted by the CIC to reset the GPIB bus. Upon receipt of this signal, all GPIB devices
on the bus will initialize themselves.
Remote Enable (REN)
GPIB devices can be controlled either locally or remotely. The CIC asserts the REN line to bring all GPIB
devices under remote programming mode. Thus, for example, the change of scale of a DVM can be
carried out by a GPIB command instead of a front panel control.
Service Request (SRQ)
Any device other than a controller can asynchronously assert the SRQ line requesting service from the
controller. The controller monitors the SRQ line and polls all devices to determine the device or devices
requiring service.
End of Identity (EOI)
The EOI signal is used by a talker to indicate the end of the data message of the talker. It indicates to the
listener(s) the end of the receiving data record.
Handshake Lines

In general, a data transfer with complete handshake gets through three stages: request or preparedness,
data transfer, and acknowledgment. On some systems, where the stability of data on the data bus is
questionable, a data valid signal may also be provided. On the GPIB bus, when a talker has to send data
to a listener, the controller must address a device and instruct it to be the talker and then address one
or more other devices and instruct them to be listeners. See later for “Addressing of GPIB Devices.”
Not Ready for Data (NRFD)
The NRFD line is controlled by the controller when sending commands or by the talker when sending
data. A device that has been instructed to be a listener will unassert NRFD to indicate to the talker that
it is ready to receive data. Of all the listeners, the slowest device will be the last to unassert NRFD and
thus control the speed of data transfer.
Data Valid (DAV)

When all listeners have indicated their readiness to receive data by unasserting NRFD, the talker (or the
controller when sending commands) will assert a DAV signal to indicte to all listeners that the data on the
data lines DIO1-DIO8 are stable and may be read by the listeners. In response to a DAV signal, the listeners

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