D
David Wisti
>Modbus/TCP seems to be everyones first
>or second choice or option but
>Modbus/TCP's only advantage over
>Ethernet/IP is simplicity but this
>comes at a great cost.
>
>1. Modbus/TCP has a very resistricted
>addressing space. 64K words is not
>enough for many applications. <
I've never found this to be a restriction in any of my applications. However, there are ways around this by using the "Unit Identifier" in the MBAP header.
>2. Modbus/TCP doesn't have data types.
>Floats, DWORDS, and DINTS are transfered
>as words. Ethernet/IP supports data
>types. <
Not true. See page 24 of the Open Modbus/TCP Specification Release 1.0 29 March 1999. There in B.2 is a chapter on data types.
>3. There should be a Modbus/UDP as well
>as a Modbus/TCP. TCP is not the best
>way to send I/O. UDP is much better.
>Ethernet/IP got this right. TCP should
>never be used for I/O. If something goes
>wrong the TCP stack will retry trying to
>send or receive old data. UDP will just
>send or receive new data the next time a
>transfer is made. This has been covered
>many times and at many forums. <
Again not true. Modbus/UDP is a well known and used by many. Kepware's KepServerEx supports it in their Modbus Suite.
>4. Modbus/TCP packets/messages are very
>small compared to Ethernet/IP messages.
>One can send a lot of data with just one
>MSG block. We have transfer 32767 words
>with one MSG block. MSTR blocks are
>limited to just 100-125 words. This
>means that a PLC program must download
>1000 words in many transfers. Many
>programmers are not capable of writing
>code that does that. <
Are you stating Ethernet/IP is simpler to code compared to Modbus?
>We have 3 Ethernet/IP certified
>products. They can communicate using
>any one of many Ethernet application
>layers including Ethernet/IP and
>Modbus/TCP. We have a pretty good idea
>of how our products being used and what
>protocol are used. Most of our customers
>use Ethernet/IP because most use
>Rockwell PLCs. Duh. Rockwell makes
>Ethernet/IP so easy and fast. Unless
>Schneider and others that use TCP have
>a bigger market share than Rockwell I
>don't see how Modbus/TCP can be
>'kicking Ethernet/IP butt'. <
Industry analysts have reported over 7 million Modbus nodes in North America and Europe alone. Why would ODVA be putting support for Modbus into Ethernet/IP if there wasn't something to gain?
Remember Modbus was created back in 1979 while it might have some limitations in 2008 it success is because its truly open, unlike Ethernet/IP.
>or second choice or option but
>Modbus/TCP's only advantage over
>Ethernet/IP is simplicity but this
>comes at a great cost.
>
>1. Modbus/TCP has a very resistricted
>addressing space. 64K words is not
>enough for many applications. <
I've never found this to be a restriction in any of my applications. However, there are ways around this by using the "Unit Identifier" in the MBAP header.
>2. Modbus/TCP doesn't have data types.
>Floats, DWORDS, and DINTS are transfered
>as words. Ethernet/IP supports data
>types. <
Not true. See page 24 of the Open Modbus/TCP Specification Release 1.0 29 March 1999. There in B.2 is a chapter on data types.
>3. There should be a Modbus/UDP as well
>as a Modbus/TCP. TCP is not the best
>way to send I/O. UDP is much better.
>Ethernet/IP got this right. TCP should
>never be used for I/O. If something goes
>wrong the TCP stack will retry trying to
>send or receive old data. UDP will just
>send or receive new data the next time a
>transfer is made. This has been covered
>many times and at many forums. <
Again not true. Modbus/UDP is a well known and used by many. Kepware's KepServerEx supports it in their Modbus Suite.
>4. Modbus/TCP packets/messages are very
>small compared to Ethernet/IP messages.
>One can send a lot of data with just one
>MSG block. We have transfer 32767 words
>with one MSG block. MSTR blocks are
>limited to just 100-125 words. This
>means that a PLC program must download
>1000 words in many transfers. Many
>programmers are not capable of writing
>code that does that. <
Are you stating Ethernet/IP is simpler to code compared to Modbus?
>We have 3 Ethernet/IP certified
>products. They can communicate using
>any one of many Ethernet application
>layers including Ethernet/IP and
>Modbus/TCP. We have a pretty good idea
>of how our products being used and what
>protocol are used. Most of our customers
>use Ethernet/IP because most use
>Rockwell PLCs. Duh. Rockwell makes
>Ethernet/IP so easy and fast. Unless
>Schneider and others that use TCP have
>a bigger market share than Rockwell I
>don't see how Modbus/TCP can be
>'kicking Ethernet/IP butt'. <
Industry analysts have reported over 7 million Modbus nodes in North America and Europe alone. Why would ODVA be putting support for Modbus into Ethernet/IP if there wasn't something to gain?
Remember Modbus was created back in 1979 while it might have some limitations in 2008 it success is because its truly open, unlike Ethernet/IP.