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IEEE 488 Introduction
Device dependant messages are only sent from the active
talker to the active listeners, they are ignored by all other
devices. A typical example of a device dependant message is the
data byte 13, HEX 0dH, when ATN line is false. This would be
interpreted by an actively listening IEEE printer as the
instruction to perform a carriage return. All the data that is sent
between IEEE devices are device dependant messages,
Interface messages are sent from the Controller In Charge
to the bus devices. Each device on the bus may accept and
process any or all depending on the capability of the device and
the type of command sent. The four groups of commands sent
by the controller are:-
1)
Universal commands accepted by ALL devices.
2)
Addressed commands accepted by all devices addressed to
listen.
3)
Addresses accepted by ALL devices.
4)
Secondary addresses or commands accepted by all devices
enabled by a primary address or command.
A typical example of an interface message would the data
byte 63, with ATN true, this is the interface message UNL, an
UNaddress, and it tells all devices to stop listening to any device
dependant data transfer.
_________ _
Addresses.
Every IEEE device has at least one address. Each device
has a unique address, often set by DIP switches. The device
addresses are used by the active controller in command mode to
specify who shall talk and shall listen. Every device must be
able to recognise its own address and respond correctly to it. No
two IEEE devices, especially talkers, should be set to the same
address.
Valid addresses are in the range 0 to 30 inclusive. It is
usual that the Talk address of a device is the same as its Listen
address.
Chapter 2
PSI BOX Reference
Page 19
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