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5522A
Operators Manual
Interface Messages (IEEE-488)
Interface messages manage traffic on the IEEE-488 interface bus. Device addressing and
clearing, data handshaking, and commands to place status bytes on the bus are all
directed by interface messages. Some of the interface messages occur as state transitions
of dedicated control lines. The rest of the interface messages are sent over the data lines
with the ATN signal true. (All device-dependent and common commands are sent over
the data lines with the ATN signal false.)
An important thing to note about interface messages is that unlike device-dependent and
common commands, interface messages are not sent literally (in a direct way). For
example, when you send a device-dependent query to the Calibrator, the controller
automatically sends the interface message MTA (My Talk Address).
IEEE-488 standards define interface messages. Table 5-4 lists the interface messages that
the Calibrator accepts. Table 5-4 also shows the BASIC statement to generate the
interface message. Table 5-5 lists the interface messages that the Calibrator sends. The
mnemonics listed in the tables are not sent in BASIC PRINT statements as commands
are; in this way they are different from device-dependent and common commands.
Interface messages are handled automatically in most cases. For example, handshake
messages DAV, DAC, and RFD automatically occur under the direction of an
instrument's interface itself as each byte is sent over the bus.
Mnemonic
ATN
DAC
DAV
DCL
END
GET
GTL
LLO
IFC
MLA
5-26
Table 5-4. IEEE-488 Interface Messages (Received)
Name
Attention
A control line that, when asserted, notifies all instruments on
the bus that the next data bytes are an interface message.
When ATN is low, the next data bytes are interpreted as
device-dependent or common commands addressed to a
specific instrument.
Data Accepted
Sets the handshake signal line NDAC low.
Data Valid
Asserts the handshake signal line DAV.
Device Clear
Clears the input/output buffers
End
A message that occurs when the Controller asserts the EOI
signal line before sending a byte.
Group Execute Trigger Trigger a TC measurement and put the reading in the output
buffer.
Go To Local
Transfer control of the Calibrator from one of the remote
states to one of the local states. (See Table 5-1)
Local Lockout
Transfers remote/local control of the Calibrator. (See
Table 5-1)
Interface Clear
A control line that sets the interface to a quiescent state.
My Listen Address
Addresses a specific device on the bus as a listener. The
controller sends MLA automatically whenever it directs a
device-dependent or common command to a specific
instrument.
Function
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