IEEE 488 Introduction
PSI BOX Reference
line only by performing a serial poll of the device requesting
service.
EOI End Or Identify.
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The EOI line, end or identify, is used by a talker to
indicate the end of a data string or, by the controller, in
conjunction with ATN, to conduct a Parallel Poll sequence.
When ATN is false, the current talker may send the END
message, indicating the end of its data, by setting EOI true at
the same time it places the last data byte on the data bus. The
controller may send the identify message, IDY, to initiate a
Parallel Poll of all devices with the Parallel Poll capability by
setting ATN and EOI true.
Negative Logic
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One of the things that commonly confuses IEEE bus users
is that the IEEE signals are described as using ’negative logic’.
This simply means that a signal is true when it is low, at 0.4
Volts or less, and that a signal is false when it is high, at 2.4
Volts or greater. Do not let this worry you, just think of the
different signals being either True or False.
Cabling Lengths and Data Transfer Speeds.
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You are permitted to connect up to 15 devices together on
the IEEE bus, that is one PSI BOX plus 14 other devices. This
should be more than adequate for the vast majority of users.
When more than 15 devices MUST be placed on one bus, the
GPIB can be extended with bus extenders.
The general limit for total cable length on the bus is 2
metres for every IEEE device connected. This is provided the
total transmission path length does not exceed 20 metres. There
should be not more than 4 metres of cable between any two
Chapter 2
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