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It should be used with caution however, since all responses are suppressed, including the
"?" error response. Thus the computer has no way of knowing if a command has been
received or even if the instrument is connected.
If a command is to be addressed to a specific instrument, but no reply is required, it is
permissible to use "$" and "@n" together. The "$" should always come first.
&
(Ampersand) instructs an instrument to ignore any following ISOBUS control
characters. It is included in the ISOBUS protocol to allow instruments whose command
repertoire includes "@", "$", "&" or "!" to be used on ISOBUS. ILM does not require the
use of this command.
!n
(Exclamation) instructs the instrument that from now on, its address is to be n. This
command is included here since it is relevant to ISOBUS operation. However for obvious
reasons, it should not be sent when more than one instrument is powered up and
connected to ISOBUS. (It would result in all instruments having the same address!) The
command is intended for initial setting up of instruments, one at a time. To avoid
inadvertently changing addresses, the "!" command will only be obeyed following a "U"
command with a non-zero password (see section 9). Note that the address set this way is
the ISOBUS address, not the GPIB address. The latter cannot be set via the interface, since
until an address is defined, GPIB communication is not possible.
6.6
The GPIB Interface
The GPIB Interface is an accessory allowing the ILM to be computer-controlled by means
of the General Purpose Interface Bus (GPIB), also known as HPIB and IEEE-488 interface.
When installed, it supplements rather than replaces the RS232 Serial Interface. It allows
an instrument to be controlled either by GPIB or RS232 (not both simultaneously). In
addition when operating under GPIB control, the RS232 interface may be used as a
GATEWAY to further OI instruments, not themselves fitted with a GPIB interface.
The instructions which follow assume some basic familiarity with the concepts of the
GPIB. This will typically be provided as part of the documentation supporting a GPIB
controller card for a computer etc.
Even with the GPIB interface fitted it is still possible to communicate with the instrument
via the RS232 interface in the standard way. This is the default condition after power up
(or t=0 re-start) and ISOBUS addressing may be used if desired.
Provided the GPIB interface has not been deliberately DISABLED by setting its address to
0 (see section 9.5), it may be switched to the GPIB IN-USE state at any time. This occurs
automatically when a GPIB Controller asserts the REN line and addresses the interface
either to talk or listen at the GPIB address selected. Once it has been put into the GPIB IN-
USE state, it remains in that state until power down or until a t=0 re-start.
