Siemens MICROMASTER 420 - Manual (Page 97)

Issue 07/04 3 Functions
MICROMASTER 420 Operating Instructions
6SE6400-5AA00-0BP0 97
NOTE
A communications (CB) module as well as a programming / operator unit can
be simultaneously connected to the COM link interface via terminals 14/15
(USS). This is the reason that the communications module has priority over
USS. In this case, the USS node (USS station) via the COM link is de-activated.
USS at the COM link (RS485)
Contrary to PROFIBUS, the RS485 port (terminals 14/15) is not optically
isolated (not floating). When installing the system, it must be ensured that EMC
faults do not result in communication failures or damage to the RS485 drivers.
The following measures should be taken as a minimum:
1) Expose the motor shield and correctly connect the shield at both ends of the
cable. If at all possible, avoid interrupting the cable. However, if this cannot
be avoided, then it is crucial that the shield at the connection locations is
continued in compliance with EMC rules.
2) All node points should be well-grounded (EMC ground).
3) Interference suppression elements should be connected to all relay coils.
4) The cables, as far as possible, should be routed separately from other
cables. Especially RS485 cables should be kept away from motor cables.
5) The shields of RS485 cables must be correctly grounded.
If the AOP communicates using the USS protocol, then contrary to the BOP, the
appropriate USS parameters must be set (Table 3-12 and Table 3-13).
In order to ensure error-free communications, the appropriate communication
parameters must be set and harmonized with one another – both in the drive
converter as well as in the connected drive unit or in the connected option
module. In this case, the relevant operating instructions should be used for the
AOP and for the communication modules.