78
The reply from an active slave is
L {N} ? A *
An inactive device will give no reply.
Type 2 Message
L {N} {P} {C} * or R {N} {P} {C} *
This type of message is used by the master device,
to interrogate or modify a parameter in the addressed
slave device. {P} identifies the parameter and {C} rep-
resents the command to be executed, which may be
one of the following:
+ (Hex 2B) = Increment the value of the parameter
defined by {P}
– (Hex 2D) = Decrement the value of the parameter
defined by {P}
? (Hex 3F) = Determine the current value of the
parameter defined by {P}
The reply from the addressed slave device is of the
form:
L {N} {P} {DATA} A * or R {N} {P} {DATA} A *
where {DATA} comprises five ASCII-coded digits whose
format is shown in the Data Element table above. The
data is the value requested in a query message or the
new value of the parameter after modification. If the
action requested by the message from the master de-
vice would result in an invalid value for that parameter
(either because the requested new value would be out-
side the permitted range for that parameter or because
the parameter is not modifiable), the slave device re-
plies with a negative acknowledgement:
L {N} {P} {DATA} N * or R {N} {P} {DATA} N *
The {DATA} string in the negative acknowledgement
reply will be indeterminate. If the process variable or
the deviation is interrogated whilst the process variable
is outside the range of the slave device, the reply is:
L {N} {P} < ? ? > 0 A *
if the process variable is over-range, or
L {N} {P} < ? ? > 5 A *
if the process variable is under-range.
Type 3 Message
L {N} {P} # {DATA} * or R {N} {P} # {DATA} *
This message type is used by the master device to set
a parameter to the value specified in {DATA}. The com-
mand is not implemented immediately by the slave de-
vice; the slave will receive this command and will then
wait for a Type 4 message (see below). Upon receipt
of a Type 3 message, if the {DATA} content and the
specified parameter are valid, the slave device reply is
of the form:
L {N} {P} {DATA} I * or R {N} {P} {DATA} I *
(where I = Hex 49) indicating that the slave device is
ready to implement the command. If the parameter
specified is invalid or is not modifiable or if the desired
value is outside the permitted range for that parameter,
the slave device replies with a negative acknowledge-
ment in the form:
L {N} {P} {DATA} N * or R {N} {P} {DATA} N *
Type 4 Message
L {N} {P} I * or R {N} {P} I *
This type of message is sent by the master device to
the addressed slave device, following a successful
Type 3 transaction with the same slave device. Provid-
ed that the {DATA} content and the parameter speci-
fied in the preceding Type 3 message are still valid, the
slave device will then set the parameter to the desired
value and will reply in the form:
L {N} {P} {DATA} A *
where {DATA} is the new value of the parameter. If the
new value or parameter specified is invalid, the slave
device will reply with a negative acknowledgement in
the form:
L {N} {P} {DATA} N *
where {DATA} is indeterminate. If the immediately pre-
ceding message received by the slave device was not
a Type 3 message, the Type 4 message is ignored.
Error Response
The circumstances under which a message received
from the master device is ignored are:
Parity error detected
Syntax error detected
Timeout elapsed
Receipt of a Type 4 message without a preceding
Type 3 command message.
Negative acknowledgements will be returned if, in spite
of the received message being notionally correct, the
slave device cannot supply the requested information
or perform the requested operation. The {DATA} ele-
ment of a negative acknowledgement will be indeter-
minate.
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