Addressing The 7470 As A Talker Or Listener; Computers With No High Level I/O Statements; Computer With High Level I/O Statements - HP 7470A Interfacing And Programming Manual

Addressing the 7470 as a Talker
or Listener
In order to communicate effectively with the 7470plotter, it is important
that you completely understand the addressing protocol of your com­
puter. Therefore, you may wish to review this aspect of your computer
before proceeding.
Computers with No High Level I/O Statements
On low level computers, addressing devices on the HP-IB bus is accom­
plished using mnemonics, such as CMD, which serve as the "bus
command."
When bus commands are necessary, a typical addressing sequence is
<Unlisten Command> <'TalkAddress> <Listen Addresses>
This sequence is made up of three major parts which serve the following
purposes:
1. The unlisten command is the universal bus command with a char­
acter code of "?". It unaddresses all listeners. After the unlisten com­
mand is transmitted, no active listeners remain on the bus.
2. The talk address designates the device that is to talk. A new talk
address automatically unaddresses the previous talker.
3. The listen addresses designate one or more devices that are to listen.
A listen address adds the designated device as listener along with
other addressed listeners.
This basic addressing sequence simply states who is to talk to whom.
The unlisten command ("?") plays a vital role in this sequence. It is
important that a devicereceive only the data that is intended for it.
When a new talk address is transmitted in the addressing sequence,the
previous talker is unaddressed. Therefore, only the new talker can send
data on the bus and there is no need to routinely use an untalk
command in the same manner as the unlisten command.
Computers with High Level I/O Statements
In more powerful computers, higher level input/ output (I/O) state
ments are used to specify device addresses on the HP-IB bus. In these
cases, the addressing protocol (unlisten, talk, listen) is a function of the
computer's internal operating system and need not be of concern to the
user.
9-6 HP-IB INTERFACING