COMMERCIAL IN CONFIDENCE
Page 9
2. ACTIONS REQUIRED TO COMMUNICATE WITH THE
INSTRUMENTS
These versions of the instruments have ModBus functions codes 1,3,5,6 and 16 implemented. Note
that although function code 16 (write multiple registers) is recognised, this cannot deal with more than
a single 2 byte register write and will return an error if more registers are attempted. There is no facility
to read multiple registers, nor to read or write multiple bits.
2.1
Notes
•
Un-implemented function codes do not yet return error code 01.
•
Function code 15 (read multiple registers) is not implemented.
2.2
Implementation restrictions
A number of restrictions are made :
•
Multiple register reads are not implemented, since the opportunity to use them is very
limited and there is not sufficient memory to buffer long messages.
•
Multiple bit reads and writes are not implemented, since we do not have any consecutive
bits available to the user.
2.3
Security Messages
To allow communications to safely manipulate the instrument a number of security messages
have been implemented. CAL Controls see these as important safety features, which offer a
number of advantages, especially when configuring a safety critical application.
The messages to enter program mode only have to be sent once to access the Internal
parameters of the instrument and then any number of adjustments can be made. The advantage
this offers is that sending the enter program mode message, causes the push buttons on the
instrument to be locked out. This feature prevents potentially dangerous conditions arising
from simultaneous adjustment of the instrument locally whilst adjustments are being sent over
the communications link.
The messages to exit program mode causes the instrument to write back any internal
parameter changes to the NVram, and then use these settings. This means that any changes
made will not take effect until the instrument has received the exit program mode message. The
advantage this offers is that all adjustments take place at the same time. If for instance you are
configuring alarm functions you will not get false alarms due to setting the alarm mode before a
valid alarm set point has been programmed, also all PID terms are implemented together,
whereas separate adjustment of PID functions may cause greater control instability.
To prevent inadvertent changes a
security byte
must be set immediately before any security message
is transmitted - this byte is automatically reset after each message.
Each security message is numbered 1 to 6, this number must be set into the
security byte
immediately
prior to the message, if the
security byte
does not match the security message number, the message
will be ignored and no response will be issued. Messages 1 to 4 are implemented but currently have no
direct use.
Note: The 9500 does not require the security byte messages to be sent to enter and exit
program mode, the messages not required are noted below.
The correct sequence to set new parameters into the instrument is as follows:
