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If a command is not recognised, has an illegal parameter or cannot be obeyed for any
reason, an error response will be sent. This consists of a "?" (question mark), followed by
all or part of the command string in question. To simplify error handling in the
computer, the "?" will always be the first character returned.
6.4
Numeric Parameters
All numeric parameters are treated as signed integers and are sent as a string of decimal
digits. The range of acceptable numbers is -32768 to +32767. Alternatively, positive
numbers in the range 0 to 65535 will be accepted, if preceded by a
"#"
(hash) symbol.
Numbers outside this range will give an error.
For positive numbers, the
"+"
sign is optional, as are leading zeros. Any spaces, full stops
and commas embedded within the number are ignored.
6.5
Use with Oxford Instruments ISOBUS
The Oxford Instruments ISOBUS allows a number of instruments to be driven in parallel
from a single RS232 port on a computer, using a special cable assembly.
To allow separate instruments to be distinguished, each is allocated a unique address in
the range 0 to 8, held in non-volatile memory.
When operating on ISOBUS, an instrument must be able to recognise and respond to
commands addressed to it, whilst ignoring commands addressed to other instruments.
This is achieved by starting all commands with a special ISOBUS control character.
When more than one powered-up instrument is connected on ISOBUS, no command
should be issued which does not have an ISOBUS control character as its first character.
Issuing such a command would result in an unintelligible response, as all instruments
would reply together. (N.B. This will only result in lost data. No hardware damage will be
caused.)
Following the control character and its parameter (where required), the rest of the
command follows the form described above. The response of the instrument depends on
the initial control character in the following manner:
@n
(At) addresses the command to instrument number n, where n is a digit in the range
0 to 8. This instrument obeys the command and returns its usual response. All other
instruments ignore the command and send no reply.
$
(Dollar) instructs all instruments to send no reply. This is normally used to precede a
command being sent to all instruments simultaneously, and prevents a conflict as they all
echo the command together.
It may also be used in non-ISOBUS applications if the computer does not wish to receive a
response.
