KTA-282 Modbus TCP Weather Station Gateway
The weather station requires the pressure to be in thousandths of an inch mercury. Therefore 1013.25 mbar
becomes 29.9 inHg; 29900 mInHg. The elevation must be in feet; 204 metres becomes 670 feet.
Write 29900 to Modbus register 115, and 670 to register 116. Commit the values to the weather station by writing
register 117 to 1.
Note: the BAR= command must send both arguments to the weather station. If one of elevation or barometer
calibration does not need to be written, you must manually write that register to zero before sending the BAR=
command.
Example (EEWR command):
The user desires the latitude to be set for Melbourne, Australia. The calculation is as follows:
Melbourne lies at 37.7° South. Southern latitudes are represented in Davis weather stations’ as negative tenths of a
degree (-377). As an unsigned number in 2’s compliment this is 65159 or FE87 in hexadecimal.
To write this figure to the weather station, two commands will have to be used – one for each byte. The latitude is
held in EEPROM location 0B in the weather station and is two bytes long. Values are
little endian (least significant
byte first)
, therefore the following two commands must be sent:
EEWR 0B 87
EEWR 0C FE
This is done with the Modbus interface as:
Write 0B [or decimal 11] to register 111 and 87 [decimal 135] to register 112. To send the command, write a 1 to
register 113. To confirm the EEPROM write as a success, read register 114. Success will be represented with a 1.
To write the second command, first write register 114 to zero (so that success can be confirmed on the next write).
Follow the same procedure as before: 0C [decimal 12] (the next address in the weather station's EEPROM) to
register 111, FE [decimal 254] to register 112, and a 1 to register 113. Register 114 will again change to a 1 upon
EEPROM write success.
This completes setting the latitude of the Davis weather station.
Note: consult the
EEPROM configuration settings
section of “Vantage Serial Protocol Docs” on the Ocean Controls
website for detailed information on the EEPROM.
Modbus TCP Manual Socket Severing
Some users have experienced some problems with their Modbus TCP connection failing after prolonged use
(between days and months). If this is a problem for you, it may help to enable manual socket severing by
writing a
one to Modbus Holding Register 125
.
Modbus Holding
Register
Name
Description
125
Modbus Socket
Sever
1 =
Automatically severs a Modbus TCP connection after
90 seconds of inactivity.
0 =
Normal Modbus behaviour (socket remains open for
the duration of the connection).
The issue is caused by the Modbus Master mishandling the Modbus connection. In this situation, the Master opens
a socket to the KTA-282 and begins communicating over Modbus TCP. At some point, the Master stops
communicating on that socket, and tries to open another socket,
without properly closing the first connection
.
Communications continue on the second socket. Again the Master stops communicating on the second socket, and
opens a third. The KTA-282 supports up to three simultaneous Modbus TCP connections, so at this point all comms
are OK. But when it happens again, the KTA-282 will refuse the connection, because all sockets are occupied. The
only way to recover is to power cycle either the KTA-282 or the Modbus Master, and manually sever the sockets.
By writing a one to Holding Register 125, you are enabling manual socket severing. The KTA-282 monitors all open
connections, and if there is no actual traffic on a socket for 90 seconds, that connection is artificially forced to close.
Webpage
In addition to providing an interface to read and write the TCP/IP configuration values, the webpage also provides
the user with a view of the current status of the KTA-282.
7/05/2013
www.oceancontrols.com.au
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