Lynn's Industrial Protocols over IP

Monday, December 11, 2006

Simulating Multi-drop Across routed IP

Summary: the UDP Sockets profile in Digi device servers can be used to simulate multi-drop behavior in routed IP or wide-area networks. An application note is linked to this entry.

Ever wished your Ethernet could mimic an RS-485 network? Or are you trying to replace an old, expensive multi-point analog modem system with newer IP-based technologies such as cellular, satellite, or aDSL links?

On a local Ethernet subnet a UDP broadcast can be used to simulate multi-drop ... however IT departments and anyone thinking of the future knows IP broadcast is something not to be used lightly. IP broadcast loads every device on the network and examples of high broadcast load killing or crippling important embedded devices are common.

The preferred method on a local Ethernet is the use of Class D IP (aka IP addresses in the 224.x.x.x to 239.x.x.x range). However, details of IP assignment, IP collision, and the risk of turning switches into hubs make this a risky and confusing technology. Most heavy users of ODVA Ethernet/IP can cite a few cases where enabling high multi-cast traffic killed other third-party products (notably security or video devices) which had treated all multicast traffic as broadcast to be examined by software.

Plus we are talking about wide-area-networks and use of cellular or satellite technology. Routed IP networks won't move broadcast or multicast traffic unless active proxies exist at each end to encapsulate the broadcast/multicast traffic into TCP/IP.

Fortunately, the Digi One IAP (and most Digi device servers) include the ability to use a form of repeated UDP/IP unicast to simulate multicast to up to 64 remote peers. I have customers using this to move Modbus/RTU and AB DF1 Half-Duplex through routed private wide-area-network. Here is an application note which explains how to set this up.

(For now it is a Word 2003 document - but it can be opened by Open Office Writer v2.0 if you don't have Word. I'll shortly turn it into a Acrobat PDF)


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