Siemens SIMATIC S7-400 Reference Manual page 25

Error LEDs and Points to Note, CPU 41x-3 and 41x-4
The CPUs 41x-3 and 41x-4 continue to have the LED IFM1F and LEDs IFM1F and
IFM2F. These indicate errors in connection with the first and second module
interfaces.
LED
IFM1F IFM2F
H
x
B
x
H = LED lights up; B = LED flashes; x = LED status is irrelevant
Diagnostic buffer
You can read out the exact cause of an error in STEP 7 (PLC –> Module
Information) from the diagnostic buffer.
Automation System S7-400 CPU Specifications
A5E00165965-01
x An error has been detected at module interface 1.
H An error has been detected at module interface 2.
x DP master: One or more slaves on the PROFIBUS DP interface
module inserted in module slot 1 are not responding
DP slave: not addressed by the DP master
B DP master: One or more slaves on the PROFIBUS DP interface
module inserted in module slot 2 are not responding
DP slave: not addressed by the DP master
Structure of a CPU 41x
Meaning
1-13