The document discusses programming programmable logic controllers (PLCs) from Java applications. It describes challenges in connecting to PLCs over a large distance, including high latency. It also discusses issues encountered using an open source Modbus library for Java and improvements made by forking the library. The document emphasizes communication challenges between different systems using different protocols and terminology.

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Low-level programming
in the factory
Stephan Rauh
OC in Argentina

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… in Argentina
Image source: Wikimedia (CC BY-SA 3.0)
Let‘s Control the Assembly Line!

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What is a PLC?
Image source: https://de.wikipedia.org/wiki/Speicherprogrammierbare_Steuerung#/media/File:Programmable_logic_controller.jpg

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Controlling
Hardware
Half a World
Remote
Image source: https://pixabay.com/de/illustrations/welt-karte-weltkarte-erde-mercator-2169041/ (Pixabay license); https://en.wikipedia.org/wiki/File:Server-multiple.svg

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Performance
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12.000 km / 300.000 km/s = 40 milliseconds
real-world latency: ≥ 75 ms
133 network calls: ≥ 10 seconds

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Programming a PLC
1. Reading input registers and coils
2. Executing the program
3. Processing communication requests
4. Executing CPU diagnostics
5. Writing output registers and coils

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Registers and Coils
Coils
little
endian
read-
only
big
endian
Registers
memory
address

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Java Modbus Adaptor
JBoss Application
Server
PLCModbus
Adaptor
Modbus
TCP
SOAP over
JMS
• 12.000 km distance
• cyclic polling

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Putting our project in perspective
Manufacturing Execution System
running on:
• JBoss Application Server
• JBoss Fuse ESB
• Active MQ
• SOAP over JMS
• Much more

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Modbus Library
How to speak Modbus/TCP?
 Implement the protocol from scratch
 Possible but tedious
 Buy a library
 There ain’t none!
 At least not for Java
 Use an open source library
 Many C++-based libraries
 … but we’re not familiar with JNI
 Found a Java library at https://github.com/klymenek/modjn

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Getting started with the Modbus Library
 Modjn offers a fairly simple API
 Got up to speed quickly
 We added a small abstraction Java-friendly layer

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Modbus Library surprises
 Most Modbus libraries have been written by engineers
 Not software developers ☺
 If it runs, it’s good
 Modjn is great for proof of concepts
 Not reliable in the long run
 Errors piled up:
 Lost messages,
 wrong messages
 Many reconnects
 Performance problems (150+ messages in a 10 second time frame)

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Modbus Library surprises
 We started to analyze and dissect the library
 Modjn offers a synchronous API using asynchronous Netty
 We could have sent the 150+ messages in parallel!
 But switching to async wasn’t an option
 We forked the library to make it more resilient
 Tried to contact the author
 No response
 Conclusion: We’re using an abandoned library!
 Luckily it’s simple
 We can maintain it ourselves
Unexpected project
risk!

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Talking past
Each Other
Image source: Wikipedia (public domain)

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Talking at Cross Purposes
0x0030
0x0060
0x0090
MVSK
NGA
LGL
ONT
moveSkid()
dropItem()
pickItem()
Image source: https://commons.wikimedia.org/wiki/File:Mailbox.svg published under a CC BY-SA 3.0 license
WebService names
Legacy telegram
names
What’s really
happening

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Mailboxes
0x0030
0x0060
0x0090
Telegram 1
Telegram 2
Telegram 3
Telegram 4
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Mailboxes
0x0030
0x0060
0x0090
Telegram 1
Telegram 2
Telegram 3
Telegram 4
Memory address content
0x0060 Command ID
0x0062 Skid Number
0x0063 Target position
0x0064 …
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Any Questions?
 It's your turn!
PLC programming for Java developers

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PLC programming for Java developers