Measurement Computing Personal488 rev.3.0 For DOS & Windows 3.Xi - Manual (Page 145)

8P. Command Descriptions II. SOFTWARE GUIDES - 8. Driver488/DRV
II-130 Personal488 User’s Manual, Rev. 3.0
Bus Management Lines
For the general control and coordination of bus activities, five bus management lines are used by either
an IEEE 488 interface or a serial interface:
•
Interface Clear ( IFC ): Employed by either IEEE 488 or serial interfaces, this line is used only by
the System Controller to place all bus devices in a known, quiescent state. Specifically, the IFC
places the devices in the Talk and Listen Idle states (neither Active Talker nor Active
Listener) and makes the System Controller the Active Controller.
•
Remote Enable ( REN ): Employed by either IEEE 488 or serial interfaces, this line is used only by
the System Controller to allow bus devices to respond to remote (bus) commands. When REN is
asserted, all listeners capable of remote operation enter remote operation when addressed to Listen.
If
REN is unasserted, then the bus devices may ignore the bus and remain in local operation.
Generally, the REN command should be issued before any bus programming is attempted.
•
Attention ( ATN ): Employed by an IEEE 488 interface, this is one of the most important lines for
bus management, and can only be driven by the Active Controller. When ATN is asserted, the
information contained on the data lines is to be interpreted as a bus (multiline) command. When it
is unasserted, that information is to be interpreted as data for the Active Listeners.
•
End-Or-Identify ( EOI ): Employed by an IEEE 488 interface, this line is used to signal the last
byte of a multibyte data transfer. The device that is sending the data asserts EOI during the
transfer of the last data type. The EOI signal is not always necessary, since the end of the data may
be indicated by some special character such as the carriage return. The Active Controller also uses
EOI to perform a Parallel Poll by simultaeously asserting EOI and ATN .
•
Service Request ( SRQ ): Employed by an IEEE 488 interface, this line is asserted by any device to
attract the immediate attention of the Active Controller. Consequently, it can be used to interrupt
the current sequence of events. The device may be reporting that it has data to send, an error
condition to report or both. The Controller can determine which device requested service using
Serial Poll or Parallel Poll. The Serial Poll will clear the
SRQ line unless some other device is
requesting service.
•
Data Terminal Ready ( DTR ): Employed by a serial interface, this line is specified to indicate the
presence and readiness of data terminal and data communication equipment (DEC). The DTR is
asserted by the terminal equipment when terminal power is on, indicating to the modem or other
DCE that the terminal is ready.
•
Ring Indicator ( RI ): Employed by a serial interface, this line indicates that a ringing signal is
being received on the communication equipment.
•
Request To Send ( RTS ): Employed by a serial interface, this line is specified to assist half-duplex
communication equipment in transmitting and receiving data. Before a transmission, the sender’s
RTS signal is asserted, requesting the receiver to switch its circuitry to the receive mode.
Handshake Lines
To “handshake” the transfer of information across the data lines, three lines are used by either an
IEEE 488 interface or a serial interface:
•
Data Valid ( DAV ): Employed by an IEEE 488 interface, this line is controlled by the Active
Talker. Before sending any data, the Talker verifies that NDAC (see below) is asserted, which
indicates that all Listeners have accepted the previous data byte. The Talker then places a byte
onto the data lines and waits until
NRFD (see below) is unasserted, indicating that all Addressed
Listeners are ready to accept the information. When NRFD and NDAC are in the proper state, the
Talker asserts DAV to indicate that the data on the bus is valid.
•
Not Ready For Data ( NRFD ): Employed by an IEEE 488 interface, this line is used by the
Listeners to inform the Talker that they are ready to accept new data. The Talker must wait for
each Listener to unasserted this line, which they do at their own rates, when they are ready for
more data. This assures that all devices accepting the information are ready to receive it.