Siemens SIMATIC S7-400H System Manual page 59

LEDs LINK1 OK and LINK2 OK
When commissioning the fault-tolerant system, you can use the LINK1 OK and LINK2 OK
LEDs to check the quality of the connection between the CPUs.
LED LINKx OK
Lit
Flashes
Dark
LED MAINT
This LED indicates that maintenance is required. For more information, refer to the STEP 7
Online Help.
Diagnostic buffer
In STEP 7, you can select "Target system -> Module Information" to read the cause of an
error from the diagnostics buffer.
S7-400H
System Manual, 03/2012, A5E00267695-11
Meaning
The connection is OK
The connection is not reliable, and the signal is disrupted
Check the connectors and cables
Check whether the fiber-optic cables are installed in accordance with the guidelines in chapter
Installation of fiber-optic cables (Page 346).
Check whether the synchronization module works in another CPU.
The connection is interrupted, or there is insufficient light intensity
Check the connectors and cables
Check whether the fiber-optic cables are installed in accordance with the guidelines in chapter
Installation of fiber-optic cables (Page 346).
Check whether the synchronization module works in another CPU.
Assembly of a CPU 41x–H
5.3 Status and error displays
59