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Modbus IP Multiple Queries and Multiple Query Packets (Async) explanation

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Modbus IP Multiple Queries and Multiple Query Packets (Async) explanation

Janeway RobertAndriolo Janeway
‎2020-11-11 03:47 AM

on ‎2020-11-11 03:47 AM

Issue

Details on how Modbus IP Multiple Queries and Multiple Query Packets work

Product Line

EcoStruxure Building Operation.

Environment

  • Building Operation SmartX Servers
  • Modbus IP

Cause

Understanding of how the Modbus synchronous and asynchronous transport layers differ.

Resolution

By default, the Multiple queries is Disabled, this is the synchronous transport layer. To enable the asynchronous transport layer the Multiple queries must be Enabled within the Modbus TCP Network properties as shown below.

ModbusTCPNetwork.png

Setting to "Enabled" allows multiple queries (packets) to be sent to multiple Modbus devices, thus reducing wait times.  It also enables further settings in the Modbus Device properties "Maximum concurrent queries" and "Multi-query packets".

 

The maximum concurrent queries and multiple-query packets as shown below are configurations for the Modbus asynchronous transport layer.

ModbusTCPDevice.png

The Maximum concurrent queries setting is only enabled if the "Multiple queries" is enabled in the Network Properties. When enabled, this allows the "concurrent queries" to be sent as a single packet.  

 

Modbus Synchronous Transport Layer

Each Modbus query is sent out one after the other. Before sending out the next query the transport layer waits for a specified period for the previous query.


Example Poll Sequence

Device No. Objects
1 2
2 1

D=Device, Q=Query, R=Reply

D1 Q1 Send
D1 R1 Receive 
D2 Q1 Send
D2 R1 Receive 
D1 Q2 Send
D1 R2 Receive 

If it takes 1 second for the device to reply, then this sequence would take 3 seconds.

Modbus Asynchronous Transport Layer

With the asynchronous transport layer enabled, the queries are transmitted to multiple devices at the same time. The maximum number of queries that are transmitted to that one device is controlled by the “Maximum concurrent queries” setting. The “Multiple-query packet” specifies whether the queries are transmitted in one TCP frame or not.

Simple Example Poll Sequence

Device No. Objects Maximum concurrent queries Multiple-query packet
1 2 False 1
2 1 False 1

D=Device, Q=Query, R=Reply

D1 Q1 Send
D2 Q1 Send
D1 Q2 Send
D1 R1 Receive 
D2 R1 Receive 
D1 R2 Receive 

If it takes 1 second for the device to reply, then this sequence would take 1 second

Complex Example Poll Sequence

Device No. Objects Maximum concurrent queries Multiple-query packet
1 10 True 5
2 4 False 2

D=Device, Q=Query, R=Reply

D1 Q1, Q2, Q3, Q4, Q5 Send
D2 Q1 Send
D2 Q2 Send
D1 R1, R2, R3, R4, R5 Receive 
D2 R1 Receive 
D2 R2 Receive 
D1 Q6, Q7, Q8, Q9, Q10 Send
D2 Q3 Send
D2 Q4 Send
D1 R6, R7, R8, R9, R10 Receive 
D2 R1 Receive 
D2 R2 Receive 

If it takes 1 second for the device to reply, then this sequence would take 2 seconds

 

NOTES

  • The use of this feature should be avoided before EBO v3.1
  • Some Modbus devices do not support multiple queries or multiple query packets, so the feature should be tested on an individual basis. (Some devices may even go offline until manually reset)
  • This feature will increase the CPU usage on the EBO server. It should be avoided on EBO Servers that are already heavily loaded
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