Alicat PROFIBUS - Manual (Page 5)

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At power up, if a user does a configuration read before sending a configuration
as is the case when auto configuring the network, this default configuration
will be sent.
If during operation, after the unit has been configured, the unit is queried for its
configuration it will send the current configuration. If a power cycle occurs the
unit will return to the default configuration until it is again configured.
4.3 Data Exchange
Once a correct configuration telegram has been received the unit will enter
data exchange mode. In this mode input telegrams will be sent whenever
new data is available and output telegrams will be processed as they are
received. If communication is lost during data exchange, the unit will return to
wait parameterization mode.
4.3.1 Data In Telegram (To Master)
The data in telegram from the unit can contain any combination of process
variables, gas string, and status byte. In the case of a controller, the PID
variables can be configured to be read to allow tuning of the control loop.
4.3.1.1 Process variables
Each of the process variables are 4 bytes long and formatted in big endian
IEEE 32 floating point. If the meter/controller doesn’t support a value it will
always be zero.
All the reading values are in the engineering units specified at time of order
(for example: with a 200SLPM controller the mass flow will be SLPM).
4.3.1.2 Gas String
Gas is a null terminated string that has 8 bytes. If the gas specified is less than
7 characters long, the gas string will be in the leading octets and be followed
by zeros (nulls).
4.3.1.3 Status Byte
The status is a byte that contains active high bits in the following bit format:
0000TMVP
Where:
T – Totalizer Rollover
M – Mass Over Range
V – Volumetric Over Range
P – Pressure Over Range
4.3.1.4 PID variables
For controllers, a special tuning configuration could be made and the values for
the PID loop gains will be reported here. They can be set using the command
module of the data out telegram.