User manual

Ethernet TCP/IP Modbus Application Protocol
33002479 06 07/2008 351
TCP/IP Specific Issues
Overview The following information describes some TCP/IP specific issues.
Broadcast/
Multicast
Although broadcast and/or multicast are supported by both IP network address and
IEEE 802.3 MAC address, the Modbus Application Protocol does not support either
broadcast or multicast at the application layer.
Schneider Electric’s Quantum PLCs use broadcast addressing because they use
ARP as the means of locating the destination node. The client interface to the
Modbus Application Protocol service on the PLC, the MSTR block, requires the user
to provide the destination IP address. Also the embedded stack does use a pre-
configured default gateway IP address in the case where ARP does not succeed.
TCP Port Number Schneider Electric has obtained a well-known system port from an Internet
Authority. Schneider Electric’s well-known system port number is 502. The Internet
Authority assigned the system port number 502 to asa-appl-proto with Dennis Dubé
as the company point of contact.
This port number allows Schneider Electric to transport various application protocols
over with TCP or UDP. The particular protocol is indicated by the value of the
proto_id parameter in the mbap_pdu. Currently the only assignment is 0 meaning
Modbus Application Protocol.