28 Installation
Modbus Communication Diagnostics
The “
Com
” LED indicates many Modbus communication conditions by lighting green, yellow, or
red. Other Modbus errors are indicated by returning a Modbus exception response to the master
and by saving an error code to the
registers.
Modbus Idle
Whenever the Modbus network is idle, the
Com
LED will stay off.
Received Packet / Sending Response
Every time the meter receives a properly formatted packet it will light the LED
green for 200 milliseconds.
Other Modbus Activity
If the WattNode meter sees packets on the bus addressed to other devices, it will
light the LED yellow for 200 milliseconds or longer if the packet duration is longer
than 200 milliseconds.
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.
0.2s
Green Off
0.2s
Yellow Off
Red
1.0sec
Y
R
Y
R
Y
R
Y
R
Y
R
1.0sec
Off
1.0sec
Red
Off
