Installation 29
Modbus Address Zero Invalid
Modbus address 0 is
reserved for broadcast
messages, so if the DIP switch is set for address zero, the
Com
LED will light red continuously
and the meter will not respond to any Modbus packets.
Modbus Address Conflict or Bus Contention
The meter displays this indication in these cases:
●
It sees unexpected data on the RS-485 bus when it is
preparing to respond to a command. This generally is due to another WattNode meter with
the same address responding first, although it could also be extra bytes from the Modbus
master or another device.
●
It starts transmitting a response, but doesn’t see the data it is transmitting on the RS-485
bus. This can happen if two devices have the same address and start transmitting at nearly
the same time. It can also be caused by a short circuit on the bus or extreme interference.
●
Your RS-485 adapter is configured for full duplex (four wire) operation instead of half-duplex.
●
Your RS-485 adapter is continuing to drive the transmit lines after sending a packet; this can
happen with older RS-232 to RS-485 adapters that require an RTS signal to transmit.
If you see this indication, make sure there are not two meters with the same Modbus address.
You may want to disconnect all but one meter to see if the problem goes away.
Invalid Modbus Packet
The meter will light the
Com
LED red for one second for any of
the following errors (the
registers will also be set,
but depending on the problem you may not be able to read register values).
●
CRC error
: this could indicate noise on the RS-485 bus.
●
Framing error
: this normally indicates a bad baud rate or noise on the RS-485 bus. This
can happen if you have the “
A-
” and “
B+
” wires swapped and your network isn’t properly
biased. Properly biased networks will transparently auto-detect that “
A-
” and “
B+
” wires are
swapped and correct.
Note:
some RS-485 PC interfaces label “A” and “B” the opposite of
the WattNode meter or just use “+” and “-” indications.
●
Buffer overrun error
: the packet was longer than 256 bytes.
●
Parsing error
: the packet could not be correctly parsed as a Modbus packet.
Invalid Request
If the WattNode Modbus meter receives a
valid packet
, but with an
invalid
request
(see below), then the meter will respond with a Modbus exception mes-
sage and store an error in the
registers. Because the packet was valid,
Com
LED will
flash green for 200 milliseconds.
Modbus Exceptions
If the meter receives an invalid request, it will reply with a Modbus exception code. In most cases,
your PC software should be able to display the code, which should help you determine the prob-
lem. For more information about the problem, check the
registers, which will provide
more detailed error codes.
●
01 - Illegal function code
○
ErrorStatus 213: The Modbus function code is not supported by the WattNode meter,
such as
07 Read Exception Status
.
Red
1.0sec
Y
R
Y
R
Y
R
Y
R
Y
R
1.0sec
Off
1.0sec
Red
Off
0.2s
Green
Off
