13
The host initiates all transactions. Slave devices continuously monitor the net-
work, looking for messages framed by silent periods. The first character detected,
after a silent period, is assumed to be an address byte and is compared to the
meter’s internal address (zero for broadcasts). An addressed slave reads the
remainder of the message and acts upon it as required.
A slave tests the message to determine its validity and uses the transmitted
checksum (CRC) to detect communication errors. A slave will only act on valid
messages, received without error, specifically addressed to it.
ADDRESS
Valid Modbus addresses are in the range 0-247. Individual devices may be as-
signed addresses in the range 1-247. Address 0 is retained for broadcast com-
mands which are handled by all slaves. When a slave responds to a command, it
places its own address in the replay message.
FUNCTION
The functions code is a single byte, telling the device what type of operation to
perform. Valid Modbus codes are in the range 1-255 decimal, but the GIMA Op-
tions Module handles only a small subset of these as summarized below.
Figure 4.7. Function Code Summary
DATA FIELD
Data from the host contains additional information for the remote device specific
to the command. For example, the data field may specify which meter readings
are required or new values for energy registers. Data from a slave may contain
meter readings or other information requested by the host. The slave also uses
the data field to send error codes.
The size of the data field varied depending on command type and usage. The
data format may also vary from one command to another to suit the application.
Instantaneous readings, for example, are transmitted as 2-byte Integers, whereas
energy readings are formatted as 4-byte Long Integers. Data is always transmit-
ted with the most significant byte first. Data formatting is described in more detail
in the following sections.
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4.5.4.2 Scaling Instantaneous/Peak Values
Instantaneous readings from the GIMA are provided as Signed Integer values
with no decimal point or legend (e.g. kW or MW). Scaling factors are provided to
enable conversion of the raw data to real numbers in basic unit form (amps, volts,
watts, VA or var). These scaling factors are constant values calculated in the
GIMA as a function of CT and PT Primary programming.
To convert raw data to real numbers:
R = I X 10
(K-3)
Where
I
= Integer number
K
= Relevant Scaling Factor
R
= Real number result
Example:
If the meter is programmed with CT Primary = 50A and PT Primary = 415V:
The meter displays would be scaled as : 50.00A, 240.0V, 415.7VLL, 36.00kW.
Scaling factors would be as
I Scale = 1, Vph Scale = 2, VLL Scale = 2, P Scale = 4.
Integer Values would be transmitted as: 5000, 2400, 4157, and 3600.
Amps would be calculated as 5000 x 10
(1-3)
= 5000/100 = 50.00A.
Phase Volts would be calculated as 2400 x 10
(2-3)
= 2400/10 = 240.0V.
Line Volts would be calculated as 4157 x 10
(2-3)
= 4157/10 = 415.7V.
3-Ph Power would be calculated as 3600 x 10
(4-3)
= 3600 x 10 = 36000W.
4.5.5 Table 14 Meter Set-up
Notes:
9. Values in Table 14 may not be written using Command 16.
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