26
ILM updates the status byte every millisecond. Thus if the status byte is read within 1mS
of reading data from the interface, the MAV and BAV bits may not yet have been
cleared, even though all available data has been read. If these bits are found to be
unexpectedly set immediately after a data read, a second read of the status byte at least
1mS later will confirm whether there really is data remaining.
6.6.4
Use of the Service Request Line
The interface will issue a service request (by pulling the SRQ line), at the point a complete
message becomes available to be read, (i.e. at the point at which MAV is first set), unless
the interface is already addressed to TALK at that point. In the latter case no service
request is required since the controller is already waiting to read the data or is in the
process of doing so.
Hence use of the SRQ line allows a suitably equipped controller to handle all data from
the interface on an interrupt basis. If the controller is not equipped to do this, it may
simply ignore the SRQ line and poll the status byte on a regular basis until the MAV bit
indicates data is available.
6.6.5
Use of the Device Clear Function
When the GPIB interface receives a Device Clear message from the controller, it responds
by clearing all the communication buffers to their empty, power-up state. It does not
reset any of the temperature control functions to the power-up state. Device Clear may
thus be safely used to empty the buffers if these have been filled with a number of
unread messages. Device Clear may be sent by either the GPIB DCL message (which clears
all connected devices), or by means of the SDC message addressed specifically to its
address.
Note that if an ISOBUS GATEWAY is in use, only the buffers in the MASTER instrument
are cleared. If data is currently being transmitted from a SLAVE instrument to the
MASTER, this will be read into the buffer after it has been cleared.
6.6.6
Use of the Interface Clear (IFC) Function
Receipt of the single line IFC message clears the GPIB interface functions as specified by
IEEE-488.1. It does not clear any pending data in the buffers. Nor does it have any effect
on operation of the temperature control function.
6.6.7
Non-Implemented Features of the GPIB
The GPIB Remote Enable (REN) line is used only to alert the interface to the presence of
an active controller. It is not used for LOCAL/REMOTE switching which is carried out by
the simpler "C" command, for compatibility with RS232 operation. Similarly the GPIB
LOCAL LOCKOUT command and GOTO LOCAL commands have no effect. This
functionality too is a part of the "C" command.
The interface does not respond to a Parallel Poll request. However since it uses open
collector data buffers, it can co-exist on the GPIB with other instruments which do have a
Parallel Poll facility.
