Rockwell and Modbus Data Traffic

Summary: I compare the data costs for four common off-the-shelf PLC protocols to a remote cellular site. As a rule of thumb, even if you have an Ethernet-based PLC your lowest data costs for SCADA-style periodic polling are obtained using serial protocols via the PLC's serial port. Their modern "Ethernet protocols" are very wasteful of cellular bandwidth.

PLC Protocol Example:
A simple but realistic SCADA scenario is to poll every 15 minutes and read 10 words of data and write 2 words of data. This commonly requires 1 Read command and 1 Write command (I'll ignore the rarely supported Modbus command that reads & writes within a single command.)

While there exists special SCADA protocols and special products that optimize remote traffic, I am not looking at those protocols at the moment. Instead, since cellular and satellite access to remote IP and Ethernet products has enabled people to use off-the-shelf PLC technology, I am looking at the more traditional PLC protocols. These are things which affect users when they apply an Ethernet design to an IP-based wide-area-network system.

I compare these 4 PLC protocols:

• AB/DF1 Radio Modem (RM) encapsulated in UDP/IP. DF1 RM is basically DF1 Full-Duplex with no ACK/NAK and is supported by the SLC5 and MicroLogix line.
• Modbus/RTU encapsulated in UDP/IP. Modbus/TCP within UDP/IP is roughly the same size.
  
• AB/CSPv4 in TCP/IP as supported by SLC5/05 and PLC5E MSG blocks.
  
• AB/Ethernet/IP as moved by ControlLogix Explicit MSG blocks to PCCC-based remote PLC. Note that Ethernet/IP "I/O Messaging" does NOT work through NAT'd wide-area-networks since the protocol embeds IP information within the data packets and is thus is "broken" by NAT.

The bytes per 15 minutes includes the IP headers, any PLC protocol overhead, and the actual data moved for each update (always 20 + 4 bytes). The MB per month is just defined by 30 days of such polling, or 2880 updates at once per 15 minutes. The 2 serial protocol have been encapsulated into UDP/IP and I assume the remote IP end-point can handle connecting this to the PLC's serial port.

Protocol	Transport	Per 15 Min	MB per month	Relative Cost
Ethernet/IP	TCP/IP	1202 bytes	3.46 MB	100%
AB/CSPv4	TCP/IP	960 bytes	2.76 MB	80%
Modbus/RTU	UDP/IP	166 bytes	0.48 MB	14%
DF1 Radio Modem	UDP/IP	194 bytes	0.56 MB	16%

The two Rockwell "Ethernet" protocols cost a lot more to use in part because they force use of TCP/IP, and therefore suffer the repeated cost of TCP socket opening and closing, plus extra TCP acknowledgment overhead. They also suffer because they both involve connection registration and service functions that needlessly repeat every time the connection is reestablished. While the actual data packets of these protocols are roughly twice the size of the serial encapsulated protocols, the real burden they suffer is all the extra TCP/IP packets exchanged that do NOT directly involve field data update.

Both the serial Modbus/RTU and DF1 Radio Modem benefit that they move no IP packets that don't relate to the field data update - no TCP/IP open or close or acknowledgement; no protocol "service function" overhead. Each moves just 1 read request and 1 read response, plus 1 write request and 1 write response.

Discussion of Other PLC Protocols:

Most other PLC Ethernet protocols will either approach the costs of the AB/CSPv4 - or they won't work at all due to use of direct "Ether-Types" and lack of IP compatibility. Most serial protocols with roughly either match the 2 show here or be twice the cost if protocol ACKs are used by the protocol.

Modbus/ASCII will almost double the cost of Modbus/RTU since each data packet is roughly twice the size. But this wouldn't increase the IP overhead any.

Using DF1 Full-Duplex instead of Radio Modem would effectively double the cost over DF1 RM since DF1 Full-Duplex moves the protocol ACK/NAK, which doubles the IP header overhead also. Using DF1 Half-Duplex would triple or even quadruple the costs since HD not only moves protocol DF1 ACK/NAKs, but the ENQ/EOT polling overhead.

Most other protocols I am aware of - such as Omron Hostlink, GE-Fanuc SNPX, and Siemens PPI - would cost roughly 2 to 3 times more than Modbus/RTU or DF1 RM since they include protocol ACK, while a few even encode many parts of the message as ASCII or BCD form instead of as binary.

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 )

(is at http://iatips.com/blogimage/rockwell_cspv4_traffic.png)

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.

Discussion
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.

DF1 Open Source for Visual Basic 2005

Archie has started a SourceForge project for his AB DF1 code running under VB 2005. He has checked in a full first set of files (unlike many SourceForge projects which get created but NEVER have files :-\ )

http://sourceforge.net/projects/abdf1/

I haven't looked over his code yet, plus all I have is VB 2003 .NET.

Hopefully Microsoft has STOPPED the old VB issue that each new rev of VB is neither 100% forward nor backward compatible ... one always need to "tweak" a few lines to make the port work. I've used VB 1.0, 3.0, 4.0, 5.0, 6.0 and now VB 2003 and none of these have liked old code being pulled forward.

Cellular to Allen-Bradley SLC5/05 on TCP 2222

The old CSPv4 protocol.

The Rockwell/AB SLC5/05 and PLC5E natively speak an older "unpublished" protocol named CSPv4, although most third party vendors call it either AB/Ethernet or AB/TCP. It moves only on TCP port 2222 - ODVA Ethernet/IP I/O Messaging is only on UDP port 2222, so they don't conflict. The protocol consists (normally) of a 3-part packet:

• 28-byte header
• 4 or 15-byte LSAP or end-point addressing packet
• PCCC message which is basically what DF1 documents as an Application Packet

In general, the packets are fairly sparse and compressible (if you have the tools to do this). The characteristics of CSPv4 which impact cellular (and wide-area-network) support:

• Rockwell tools and PLC only support use of TCP/IP and port 2222; this greatly limits use of CSPv4 in NAT'd networks since the remote NAT router can only forward TCP port 2222 to a single remote PLC.
• CSPv4 includes a single TCP packet exchange to "register a session" or connect. If you are polling faster than the PLC will hang-up on you, then this is not important. However, if you poll slow enough that a new TCP/IP socket must be opened for each poll, then even ignoring the TCP socket open/close overhead this nearly doubles your traffic costs.
• In tests, a SLC5/05 seems effective at including the TCP ACK response to the host within the CSPv4 data response packet, so you only have to pay for one empty TCP ACK, which is the host's acknowledgment to the PLC for the response.
• TCP Keepalive could be an issue, since most hosts fail to issue it and the SLC5/05 I've tested against either doesn't issue TCP keepalives
  or does it very frequently.

Interested in Cellular? Setup a DMZ Lab

Summary: Last post I suggested people interested in cellular data start by learning how to use their home cable router. In this post I suggest the next step, of how to make your life easy at work once you're ready to start testing real cellular or satellite access by IP.

Your Second Step should be to set up a simple, isolated low-speed broadband link at work ... create your own DMZ lab.

Sigh - I waste so much time listening to customers complain about how difficult it is to get the IT department to give them custom firewall permissions. Since modern "Security" wisdom is to block everything until proven safe, I waste more time asking customers complaining that their Modbus/TCP or Rockwell access not working to first talk to their IT group to make sure they aren't blocking unknown binary traffic by default. I waste yet more time when customers struggle for days and finally have to formally get someone in IT to help study the corporate firewall logs to see if any traffic is getting through or not. An interesting epiphany occurs when I suggest they just look into paying roughly $50 per month for a private connection for this. It is surprisingly cheap to do this and makes a lot of people's jobs 200% easier.

So far the feedback from customers has been quite positive, with IT departments over-joyed at the idea (slight exaggeration :-] lessor-of-two-evils may be a better term). This really makes sense; IT is charged with keeping the corporate system working and secure, so when you ask for yet another odd, unknown firewall hole to be opened, you ask them to risk their jobs. Plus trying to keep custom firewall settings updated for 50 different projects is an ongoing headache and ongoing risk for mistakes. I know that Digi's IT group is very satisfied with their policy of not offering custom firewall rules on the corporate LAN but instead helping teams set up such private connections in a safe, isolated manner.

Simple DMZ Lab Design
The simplest lab design is little more than a copy of what you have at home: a computer or two, an 8 or 24-port Ethernet switch, and a simple NAT router to "share" the internet connection with a dozen devices. This allows you to freely set up a few OPC servers and Master PLC to test access to remote cellular and other wide-area-network based systems.

• Locate an empty office or lab room for your new network. Perhaps your IT people should pull out or disable the corporate Ethernet in this room. You are going to create a small "DMZ"; a small isolated network that has limited security consequences if you goof up and let a hacker inside. You'll want good security tool installed on your Windows and Linux computer used in here.
• Arrange for a low-speed business broadband link with one fixed IP address. 256Kbps is more than enough for general PLC/SCADA testing and should cost in the range of $35 to $50 per month. Yes, just $35-50 per month! I had one customer forced to pay his IT department $100 per month to open one TCP/IP hole in the corporate firewall!! Gee, he could install 2 DSL links for that. Now, be patent when you talk to your carrier, as they are geared to sell the expensive primary access lines used for all corporate traffic including servers. Keep stressing that you want a low-speed secondary line for use with some network testing and eventually you'll locate the low-cost plans you want.
  
• Set up a DNS name for your DMZ lab. Online dynamic DNS providers support user-selected DNS names for static IP addresses. I use dyndns.org for both my dynamic and static IP but there are many out there. You won't need any form of DDNS update client since your IP never changes and you must enter the name manually anyway.
• Unless you plan to implement large VPN systems, just buy a nice consumer-grade DSL/Cable Router ... the same kind you use at home is fine. If you plan to set up a serious VPN infra-structure, then you'll just need to bite-the-bullet (& suffer the learning curve) of buying a commercial IT-grade router with VPN server capability built in. Be warned that while many consumer-grade routers mention "VPN Support", they are in fact sub-optimized and documented only for home-office users who connect into a corporate Windows or Cisco VPN server. Normal human beings will find them nearly impossible to set up for anything else!
  
• Do you want more than one public IP address? You need to pay a monthly surcharge which varies greatly per carrier, but could be in the range of $25 per month for 8 IP addresses instead of just 1. Plus you will need a larger IT-grade router since the consumer-grade routers won't support more than 1 public IP address. Most users won't need more than 1 IP address. However, having more than one IP address is helpful if more than 1 team shares the lab; this prevents them from trying to setup conflicting router configurations. Also, a few extra IP are helpful if you want to place a PLC "online" for your customers or sales force to access during customer-site demonstrations in the field.
  
• If you need to access your corporate network from your DMZ lab, then you need to arrange some rules with your IT people. Perhaps the rule is using a notebook computer with 802.11 wireless to the corporate network is Ok as long as the notebook is NEVER connected to the lab's Ethernet. Remember, since your lab has it's own public IP address you can even use FTP or a VPN client to connect "legally" out your corporate network and back into your lab via the Internet.
  

Fancier DMZ Lab Design
Since Digi is basically a "communication company", the lab I get to use is much fancier. It has 32 public IP addresses and even limited secure access from the corporate LAN. Of course I have to share this lab with other teams, so I'm not owner of 32 IP. As an example, here is how my lab is setup:

• Digi's IT group maintains a Cisco PIX router (a mid-range $800 model) that manages the 32 public IP addresses. Actually, this is NOT a complication since this router does not by default firewall any traffic; it merely distributes raw traffic based on public IP to one of many to internal IP addresses in an organized manner. I was lucky enough to get in the lab early and to be assigned 2 of the 32 public IP addresses.
  
• My first IP address receives 100% raw internet traffic at an internal static IP I selected; so an external IP such as 70.x.x.140 forwards to my internal IP of 192.168.20.159. Since the goal of the lab is to avoid burdening IT with TCP/UDP port forwarding chores, one could place a consumer-grade DSL/Cable router at this IP. Placing 2 routes in series is NOT a problem - do a net-trace of how you access www.google.com and you'll see a dozen or more routers in series. Instead of a pure hardware box I have a Ubuntu Linux machine running firewall and router tools at this IP. This is where I forward Modbus/TCP to one PLC and Rockwell Ethernet/IP to another PLC. I prefer the Linux box to the $39 hardware box because it gives me a richer view of traffic in and out, plus I can run an Ethernet sniffer such as WireShark to see a complete trace of the 2-way conversation taking place.
  
• For my second IP address I had Digi IT setup the PIX router to just forward a simple, safe list of Modbus, Rockwell, Digi, and other industrial protocol TCP and UDP ports. I normally have a Digi One IAP (an industrial-protocol aware Ethernet-to-serial device) at this IP address, but I can safely swap in a Windows machine when a test requires use of Windows tools.
  

Just as your home router does, the main Digi IT-supplied PIX router does out-going NAT to the internet and internal DHCP address assignment. So our DMZ lab has its own internal subnet of 192.168.20.x addresses. Any device in the DMZ lab can access the Internet - with or without going through my Linux firewall/router. Of course the other teams in the lab don't go through my router, but direct to the PIX router.

Since i study cellular usage, I have have a Digi Connect WAN providing an Ethernet-based cellular router in the DMZ lab. So I really have 3 potential routers to use - while it takes a bit of IP experience to not get confused, this allows me to have devices configured to selectively treat any 1 of the 3 routers as "the default gateway/router".

• For example, I can have a Master/Client device connect out to a cellular-based Slave/Server using a route such as Master => out PIX+DSL => in Cellular => Slave.
• For example, I can have a Master/Client device connect via cellular to a DSL-based Slave/Server using a route such as Master => out Cellular => in DSL+PIX => in Linux-Box => Slave.
• In both of this situations I can see BOTH ends of the conversation, which is a huge help in testing, timing, or troubleshooting new applications.
  

The bonus for having Digi's IT team involved is the PIX router also allows controlled, secure access into the DMZ lab from our corporate LAN. Technically, the PIX runs a set of rules similar to those used by the main corporate firewall out to the Internet and it just treats the 192.168.20.x subnet as a miniature internet. So I can sit at my desk and safely check on equipment running in the DMZ lab. Of course this is limited to equipment treating the PIX as the default router - if you understand IP routing you'll understand why, but at least it lets me log into my Ubuntu Linux router from desk.