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14.1.2 CIU 202 Modbus communication faults
Fault
Possible cause
Remedy
1. The slave does not respond
to telegrams.
a) Configuration or wiring error.
• Check the visual diagnostics on the Modbus
slave. Is the Grundfos GENIbus LED flashing
green and the Modbus LED off or flashing
green?
• Ensure that the cable between the Modbus
master and the Modbus slave is connected
correctly. See section
for wiring recommendations.
• Ensure that the slave address is configured
correctly, and that the correct slave address is
used in the Modbus master poll. See section
for slave address
selection.
• Ensure that the transmission speed and stop
bit/parity settings are configured correctly in
both master and slave.
• Ensure that each end of the Modbus trunk cable
is terminated, if necessary. See section
for line termination of
the Grundfos slave.
• Ensure that the bus topology for a Modbus
network is correct.
b) The slave may be in listen-only mode.
Either send a restart communications diagnostics
command, or restart the Grundfos product
manually.
c) If the holding register of address 00001
"SlaveMinimumReplyDelay" is set too high,
the master may time out before receiving the
response from the slave.
Increase the timeout span in the master in order to
communicate.
2. The slave responds with
exception response 0x01:
"Invalid function".
a) The master is trying to use an unsupported
function in CIU 202.
See section
8. Modbus function code overview
for
supported function codes. Note that reading and
writing coils are not supported, so only register
functions and diagnostics will be valid.
3. The slave responds with
exception response 0x02:
"Invalid data address".
a) The master is trying to read or write an invalid
data address. If a master tries to read register
addresses that are not listed in the tables, the
slave responds with this exception response.
Some masters may automatically try to read
large blocks in one telegram, which will cause
problems if some of the registers in the block
are not supported. An example would be
reading the CIM configuration and CIM status
blocks in one telegram. This is not possible
since there are unused addresses between
the blocks.
• Avoid reading or writing invalid data addresses.
• Ensure that register X is addressed as X - 1 in
Modbus telegrams, according to the Modbus
standard.
b) The register address offset may have been
changed from default.
Read the holding register at address 00002
"Register Offset" to see if this value is different
from 0. If so, write the value 0 to this address to
make the slave return to the default used in this
functional profile.
4. The slave returns data
value 0xFFFF (65535).
a) The value is unavailable. A data value of
0xFFFF does not necessarily indicate an error
condition. It means that the value is
unavailable from the Grundfos product.
See section
for
available data.
b) The Grundfos product is not configured to
show the value or lacks a sensor to read the
value.
See section
for data
values that require a sensor.
5. The slave does not change
Modbus transmission speed
with register 0004.
a) Configuration error.
Set the transmission speed switches to "Software-
defined". Otherwise, the value in register 0004 is
ignored by the slave.
b) An invalid value may be set in register 00004. See section
5.1 Setting the Modbus transmission
for invalid values, and set correct value in
register 00004.
