HP 3478A Technical Manual page 119

A-1. Introduction
A-2. This appendix contains a general description of the
Hewlett-Packard Interface Bus (HP-IB). HP-IB is
Hewlett-Packard's implementation of IEEE Standard
488-1978, "Standard Digital Interface for Programmable
Instrumentation". The information is non-controller
dependent but, where appropriate, is dependent on the
3478A.
A-3. General HP-IB Description
A-4. The Hewlett-Packard Interface Bus (HP-IB) is a
carefully defined interface which simplifies the integration
of various instruments, calculators, and computers into
systems. The interface provides for messages in digital
form to be transferred between two or more HP-IB
compatible devices. A compatible device can be an
instrument, calculator, computer, or peripheral device
that is designed to be interfaced using the HP-IB.
A-5. The HP-IB is a parallel bus of 16 active signal lines
grouped into three sets, according to function, to
interconnect up to 15 instruments. A diagram of the
Interface Connections and Bus Structure is shown in
Figure A-1.
A-6. Eight Signal lines, termed as DATA Lines, are in
the first set. The Data Lines are used to transmit data in
the form of coded messages. These messages are
used to program the instrument function, transfer
measurement data, coordinate instrument operation, and
to manage the system. This allows you to set-up the
instrument and read its measurement data. Input and
Output of messages, in bit parallel byte serial form, are
also transferred in the Data Lines. A 7-bit ASCII code
normally represents each piece of data.
A-7. Data is transferred by means of an interlocking
"handshake" technique which permits data transfer
(asynchronously) at the rate of the slowest active device
used in that particular transfer. The three DATA BYTE
CONTROL lines coordinate the transfer and form the
second set of lines.
A-8. The remaining five GENERAL INTERFACE
MANAGEMENT lines are used to manage the devices
on the HP-IB. This includes activating all connected
devices at once, clearing the interface, and others. A
condensed description is available in the Condensed
Description of the Hewlett-Packard Interface Bus
Manual, -hp- part number 59401-90030. The manual is
available through your local -hp- Sales and Service
APPENDIX A
Office.
A-9. HP-IB System Overview
A-10. The following paragraphs define the terms and
concepts used to describe HP-IB (Bus) system
operations.
A-11. HP-IB System Terms
a. Address: The characters sent by a controlling
device to specify which device will send information on
the HP-IB and which device(s) will receive that
information. Addressing may also be accomplished by
hardwiring a device to only send information or only
receive information.
b. Byte: A unit of information consisting of 8 binary
digits (bits).
c. Device: A unit that is compatible with the IEEE
Standard 488-1978.
d. Device Dependent: An action a device
performs in response to information sent over the HP-IB.
The action is characteristic of an individual device and
may vary from device to device.
e. Polling: This process typically is used by a
controller to locate a device that needs to interact with
the controller. There are two types of polling, as follows:
1. Serial Poll: This method obtains one byte of
operational information about an individual device in
the system. The process must be repeated for each
device from which information is desired.
2. Parallel Poll: This methods obtains
information about a group of devices simultaneously.
The 3478A does not respond to a Parallel Poll.
A-12. Basic Device Communication Capabilities
A-13. Devices which communicate along the interface
bus can be classified into three basic categories:
a. Talker: Any device that is able to send information
over the HP-IB when it has been addressed. Only one
talker may be active at a time; usually the one that is
currently directed to send data. All HP-IB type
calculators and computers are generally talkers.
A-1
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