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Section 4
Interface Control Operations
You have seen that the HP 82160A HP-IL Module gives you the capability to perform printer and mass
storage operations by using functions specifically designed for these applications. However, the Hewlett-
Packard Interface Loop is a general-purpose interface. A third set of functions—the interface control
functions—is designed to give you more complete control of interface activity—for any types of HP-IL
devices connected in the loop.
It will be helpful to digress for a moment to give a brief explanation of the operation of the interface loop.
This information should give you additional insight into how the devices interact and allow you to work
with the loop more effectively—especially when you use the interface control operations described in this
section.
Operation of the Hewlett-Packard Interface Loop
When you execute any of the functions described in this manual, the interface module translates the
function into a sequence of HP-IL instructions. It then sends them around the interface loop to each device,
one at a time. In the discussion that follows, you will learn how the calculator and peripherals
communicate using the HP-IL instructions.
Roles of Devices
In order for the interface loop to operate in an orderly manner, the devices in the loop must operate
according to their assigned roles. The role of each device is changed to suit the operation being performed.
Three different roles are defined for HP-IL devices: controller, talker, and listener. Any device not
assigned one of these roles is inactive.
The controller is the one device in the loop that can designate the roles of devices and control the loop's
operation. The system controller (the calculator) is the device that controls and initializes the loop when it
is first turned on. It can transfer control to another device, which then becomes the controller of the loop—
the active controller. Similarly, an active controller can transfer control to another device. For all
operations provided by the HP 82160A HP-IL Module, the calculator is always the system controller and
the active controller.
A talker is a device that sends information to the interface loop. It is designated and enabled by the
controller. At any time, there can be no more than one talker. The controller may be a talker. Examples of
talkers are a mass storage device sending data from a stored file and a voltmeter sending voltage
measurements
.
A listener is a device that receives information on the interface loop. There may be more than one listener
in the loop at the same time. Listeners are designated by the controller. The controller may be a listener.
(A device cannot be a listener and a talker at the same time, although it can have these roles at different
times.) Examples of listeners are a mass storage device receiving and storing data in a file and a printer
receiving and printing information.
Device Addresses
In order to distinguish between devices in the loop, each device must have an address—a number from 1
to 30. The controller uses the addresses to specify and control the devices.
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