6.1.16 C21 - Abnormal communication between IDU and ODU
Panel, display box, and wired controller
Faulty IDU: The fan continues running, and the EXV is closed. Other IDUs of the same system: The fan
continues running, the EXV is closed, and error code "A51" is displayed (V6 platform IDU displays the code
"Ed"). Meaning of the code: ODU fault
ODU of the same system:
stops.
Error code "C26" is displayed (V6 platform ODU displays the code "H7"). Meaning of the code: IDU qty
decrease fault
If the IDU has not received any communication signal from ODU for 3 min
See the Troubleshooting section.
If the indoor and outdoor units communicate via RS-485(PQE/PQ):
C21 (6)
If the indoor and outdoor units
communicate via RS-485
(PQE/PQ)
Cause 1: P/Q/E communication
cable short-circuits or cuts off (1)
Properly connect the
cable
Cause 2: Communication cables
are not connected in a series
Connect the cables in a
series
Cause 3: Cable P or Q is
connected to port E
Connect P/Q/E to the
right port
Cause 4: The communication
cable does not have a shield
layer
Use shielded cables
Cause 5: Interfered by
strong-current power cables
(over 220 V)
Separate the
communication cable
from the strong-current
power cable
Cause 6: Interfered by
electromagnetic radiation source
(transformer/high-power
fluorescent lamp, etc.)
Eliminate sources of
interference or add one
more shield to the cable
Cause 7: Different
communication networks (M1M2,
PQE, X1X2) are connected
Connect the
communication cable to
the right port based on
the type of the
communication network
Cause 8: The IDU main control
board or ODU main control board
is damaged
Replace the damaged
main control board
Note 1: If you measure the resistance between ports P, Q, and E of the IDU main control board, normally
the resistance between P and Q is 120 Ω, the resistance between P and E is infinite, and the resistance
between Q and E is infinite.
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