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1. Modbus RS485
In One Video
Saif Ali

2. Modbus RS485 Network
Siemens
Allen Bradley
ABB
etc.

3. Modbus
• Still Most Popular Protocol
• Most Supported
• Theoretical and Practical
• Modbus RS485
• Marketable and Valuable

4. History of Modbus Protocol
• Distributed Control System (DCS)
Expensive to Purchase & Maintain
Lots of Cabling
High Maintenance

5. Modicon
• Programmable Logic Controller (PLC)

7. 1979
• Modbus
• Modicon + Fieldbus = Modbus

9. Modbus
• FREE Open Standard
• Rapidly Adopted
• Many Different Manufacturers

10. History Of RS485
• USB
• RS232
RS232

11. RS232 Drawbacks
• Point-to-Point Standard Only
• Limited Distance - 50 Feet
• Susceptible to Electrical Noise

12. RS485 Features
• Point-to-Multipoint – Up to 32 Devices without Repeater
• 4000 feet
• Very Resistant to Electrical Noise

13. Common Misconceptions
• Modbus is a Protocol
• RS485 is an Electrical Standard
• Modbus can in fact use many different types of electrical
standards and is in NO way tied to RS485
• RS232, RS422, RS485, Radio, Satellite, Microwave, etc.
• So Why Modbus RS485?

14. Design Problem
PLC1 PLC2
PLC3

15. RTU vs ASCII
Modbus modes:
• Modbus RTU
• Modbus ASCII
Modbus RTU faster than Modbus ASCII (Half a Time)
Modbus RTU
Modbus ASCII

16. Modbus Device Components
• Example devices that support Modbus:
• Programmable Logic Controllers (PLCs)
• Remote Terminal Units (RTUs)
• Loop Controllers
• Power Meters
• Motor Controllers
• Pump Controllers
• etc.

17. Minimum components required:
• CPU / Microprocessor
• Memory
• Communication Interface

18. Memory Address vs. Value
• Data stored in Blocks

19. Memory Block
Memory Address
Memory Value

20. PLC – 8 Analog Inputs
PLC AI1
AI2
AI3
AI4
AI5
AI6
AI7
AI8
32100
+56

21. Memory Block Sizes
• Memory Sizes
• For Different Purposes
• Measured in Bits
• Bits = Binary + Digit >>> Can be 0 or 1
• More Bits = Higher Number
• 1 Bit
• 8 Bit
• 16 Bit
• 32 Bit

22. Memory Block Sizes
PLC DI1
Discrete Input – ON or OFF
0100
AI1
11101000
1
232

23. The Modbus Memory Map
• Modbus Memory Area
• 4 Areas
Coils
Inputs
Input Registers
Holding Registers
1 Bit
16 Bit
1 - 10000
10001 - 20000
30001 - 40000
40001 - 50000

24. PLCs back then
• Discrete Outputs
• Discrete Inputs
• Analog Inputs
• Analog Outputs
Coils
Inputs
Input Registers
Holding Registers

25. Modbus Memory Map Exceptions
PLC
AI1
30010
30006
Calculated Value

26. Determining I/O Mapping to Memory
PLC AI1
AI2
AI3
AI4
AI5
AI6
AI7
AI8
30007
31008
30020

27. Application to our Design Problem
PLC1 PLC2
PLC3
Pressure: AI3
Temperature: AI6
AI3 -> 30019
AI6 -> 30036
Valve 1: DI8
Valve 2: DI9
DI8 -> 10006
DI9 -> 10007

28. Messaging Method
PLC-A PLC-B
Request
Response
Modbus Master Modbus Slave

29. One Master – Many Slaves
Master
Slave 1 Slave 2 Slave 3
Always the Master

30. Modbus Unit ID
Master
Slave 1 Slave 2 Slave 3
Must have unique assigned identifier = Modbus Unit ID, Range 0 - 255

31. Set Modbus Unit ID
Programming software

32. All devices receive network messages
Master
Slave 1 Slave 2 Slave 3
10
21 22 23
Request
22
21 = 22 No 23 = 22 No22 = 22 Yes
Response

33. Modbus Request Commands
• Read Coil Status
• Read Input Status
• Read Holding Registers
• Read Input Registers
• Force Single Coil
• Preset Single Register

34. Modbus Block Reads
Master
Slave 1 Slave 2 Slave 3
10
21 23
Request
22
Response
22
10050
10057
Read Input Status
Modbus Unit ID = 21
Block Address
= 10050 & 10057

35. Modbus Block Reads
Master Slave 1
10050
10051
10052
10053
10054
10055
10056
10057
Read Input Status
Modbus Unit ID = 21
Start Address = 10050
Number of Blocks (length) = 8

36. Application to our Design Problem
PLC1 PLC2
PLC3
Master
Slave Slave
Pressure: 30019
Temperature: 30036
Valve 1: 10006
Valve 2: 10007
22 21
10

37. Read Commands
• PLC 2 – Memory blocks 10006 and 10007
• Read Input Status
• Modbus Unit ID = 21
• Start Memory blocks address = 10006
• Number of blocks = 2

38. Read Commands
• PLC 1 – Memory blocks 30019 and 30036
• Request #1
• Read Input Registers
• Modbus Unit ID = 22
• Start Memory blocks address = 30019
• Number of blocks = 1
• Request #2
• Read Input Registers
• Modbus Unit ID = 22
• Start Memory blocks address = 30036
• Number of blocks = 1

39. Physical Connection
• Example RS485 Port
+
-
+
-
+
-
+
-
+
-
Device 1
Device 2
Device 3
Device 4
Pair of Wires forms a Fieldbus

40. Port Settings
• RS485 Port
• Baud rate
• Number of data bits
• Number of stop bits
• Parity
+
-
Every single device on the same RS485
network
Must All have the exact same settings
for their port parameters
• 1200 bps
• 2400 bps
• 4800 bps
• 9600 bps
• 19200 bps
• 1
• 2
• 7
• 8
• None
• Odd
• Even

41. Assign Devices
+
-
+
-
+
-
+
-
Device 1
Device 2
Device 3
Device 4
Baud rate: 9600 bps
# of Data bits: 8
# of Stop bits: 1
Parity: None

42. Application to our Design Problem
PLC1 PLC2
PLC3
Master
Slave Slave
Pressure: AI3
Temperature: AI6
Valve 1: DI8
Valve 2: DI9
22 21
10
Baud rate: 9600 bps
# of Data bits: 8
# of Stop bits: 1
Parity: None

43. Thank You
By Saif Ali