Thursday, April 19, 2007

Rockwell PLC5E and SLC5 over Cellular

Summary: Customers ask me "How much will cellular cost" - this blog post walks through some examples of SCADA-style periodic polling of AB/PLC5E or SLC5/05 using the CSPv4 protocol (aka AB/Ethernet to 3rd parties) over TCP port 2222.

Real-World Numbers
For a simple SCADA-style example assume we need to read 10 words of data (20 bytes) and write 2 words (4 bytes) every time period. Obviously there would be simple optimizations to this, such as only writing data which changes or using PLC MSG blocks to push data from PLC to SCADA only when something changes. However my goal in this blog post isn't to "tweak" a solution to minimize cost, but to examine the protocol impact of using Rockwell CSPv4 over IP.

The table below shows the megabyte per month when polling once per second, per 5 seconds, per 1 minute, per 5 minutes, per 15 minutes, and per 1 hour. There are lots of variables considered ... and many more ignored. The traffic ranges from worst-case of 1005.0 MB for TCP/IP with larger header options polled once per second to best case of 0.2 MB for UDP/IP polled once per hour. This assumes the use of the CSPv4 submode 7, with local LSAP addressing and ignores that Rockwell PLC5E and SLC5/05 don't support CSPv4 within UDP/IP. Raw efficiency at moving the data bytes ranges for about 10% for UDP/IP to barely 1% for TCP/IP; which means most of what you are paying for is not related to actual, meaningful field data.

( Click this image to see a larger version )
CSPv4 Poll Record
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Notes on the Table

Since this example reads and writes small amounts of data, it assumes a SLC5-style Protected Typed Read with 3-Address Fields and the corresponding SLC5 write.

The smaller 40 byte TCP/IP header has no options attached; the larger 52-byte TCP/IP header includes the RFC 1323 Timestamp and Window Scale TCP options. These appear to be the normal default for Linux and easily becomes enabled under Windows since all applications share a single setting in the Registry.

The two time columns "15 min (Alive)" and "1 hr (Alive) assume a roughly 4 min 45 sec TCP keepalive to prevent the socket from closing. This reduces the traffic by the extra open/close overhead in exchange for billable TCP Keepalive packets. Keep in mind this ALSO requires the PLC to be properly configured to NOT close the idle sockets. By default, my SLC5/05 seems to close the idle connections in a few minutes.

The two time columns "15 min (Cls)" and "1 hr (Cls) assume the socket is closed after the a data polls, and the TCP socket and CSPv4 session must be reopened for teh next poll.

Of course the standard costs of using TCP/IP verse UDP/IP apply:
  • TCP/IP uses larger headers, ranging from 40 to 52 bytes per packet as compared to UDP/IP's smaller 28 byte of header.
  • TCP/IP involves the TCP Acknowledgments, which may result in separate, billable 40 to 52 byte packets moving frequently without any meaningful field data.
  • TCP/IP may require reopening a socket, costing 120 to 250 bytes per open, plus closing costing from 160 to 400 bytes. Exact sizes are hard to predict since both opening and closing of sockets tend to be "pushed" and result in excess retransmissions and retries when high network latency is true.
  • TCP/IP over unknown 3rd party wide-area-network infrastructure requires at least 1 TCP packet to move every 4 minutes 45 seconds to maintain health. This means either a data packet or a TCP Keepalive with data.
It should be clear to see why using UDP/IP over cellular (which is very reliable) is much cheaper than using TCP/IP. There are no socket open, close, acknowledgment, or keepalive costs. Plus field experience has shown that rapid IP retries rarely succeed. For example, a customer polling with UDP every 5 minutes with 3 explicit retries if no answer will likely have the original poll succeed or that poll and all 3 retries fail. Again, cellular is very reliable in that packets almost always make it through unless there is a network or congestion issues and then only time (a few minutes) solves the problem. So such customers have abandoned retries and just ignore 1 failed 5 minute poll and "retry" in 5 minutes.

CSPv4 issues include:
  • Rockwell PLC and software tools do NOT support use of UDP/IP - my tests with UDP/IP have to be conducted with the Digi One IAP which happily bridges CSPv4 between TCP and UDP (as well as to or from Ethernet/IP and DF1).
  • CSPv4 requires the exchange of a pair of 28-byte negotiation TCP packets when a new TCP socket is opened to inform the client (master) of a server (slave) assigned session handle. This nearly doubles the overhead of an open-poll-close socket paradigm.
  • The 28 byte CSPv4 header really contains little useful information; such excess bytes cost nothing tangible under Ethernet but cost cash in the form of requiring larger cell plans over cellar.
In conclusion, CSPv4 will be a rather poor choice for raw, periodic polling of remote AB PLC. ODVA Ethernet/IP (as implemented by all vendors) is even worse.

Your only effective solution at present is to carefully craft a set of MSG blocks to push data from the field in a report-by-exception paradigm. Of course you also must include safe guards within your PLC to prevent rapid, repeated MSG block triggers during system failure that could cost you thousands of dollar ($$$) in a few days.

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