Mitsubishi Electric MELSEC-L Series User Manual page 82

(7) When an output is turned on, a load connected to the other output is also
turned on (transistor output (source type)).
If the wire connecting 0V of an external power supply and a common of a load is cut off or disconnected, a current flows to the load that is
off due to a parasitic circuit of the output element that is off.
Cause
If a current keeps flowing under the above condition, a failure may occur.
Connect the external power supply and loads correctly.
To prevent the condition described above, connect a diode to each output terminal as shown below.
Action
80
Case
Transistor
output module
Source output
Output
element
Output
ON
control
circuit
Output
element
Output
OFF
control
circuit
Source output
Y0
Load
Y1 Load
Switch off
COM
or
disconnection
24V
0V
Y0
Load
Y1 Load
COM
24V
0V