Increasing data update rates for an IC3 Modbus controller

Issue

The time taken for values to change on a Sigma client is a lot slower then the change of values for the Modbus devices, how can this be sped up?

Product Line

Satchwell Sigma

Environment

  • Sigma Modbus
  • RS-232
  • RS-485
  • TCP502

Cause

Modbus protocol is a polling type and configuration changes to default settings can be made to speed up data collection for the IC3 Modbus controller.  

Resolution

There are changes that can be made to default settings that can allow the IC3 Modbus to collect data at a quicker or a more efficient way.  It should be noted that these changes can be counter productive and should not be applied without careful thought to the consequences.  That is, the third party Modbus device may not respond quickly enough for the IC3 Modbus request and therefore cause a retry, particularly on RS485 LANs.

1. For RS485/232 communications LAN it is important to check the LAN installation is correct.  See Lessons Learned Article #5204 particularly if Modbus controllers are going offline. 

Note:  Modbus is a polling protocol and failures and subsequent retries will slow down the update rates for those devices online.

2. Changes to the IC3 Modbus gateway.txt file;

    a) Increase baud rate

; Default Baud Rate (e.g. 1200)
9600

If the third party device (RS485) supports it, the baud rate can be increased.  But, if the communications environment is poor then reducing may be the better alternative.

   b)  Retry counters and timers

; RTU message separation time
18

-  The delay to the next IC3 Modbus request following a previous reply.  Should be increased to 200 or more to prevent timeout

; Retry Check Period
1

- Time in seconds after a failure. Suggest 1 second

; Number Of Retries
3

- Number of times to perform a retry before moving on to the next register.  Suggest 3.

; Device Reset Interval
0x01

; Device Down Interval
0x01

 - Set to 0x01 is the retry time once a failure has occurred and before the next time it will attempt device again (0x01 = 60 seconds)

   c) Delay between controllers

In the body of the mapping table the 5th field is the minimum time before polling the same device and can be set to zero

31,2,3,0,1,0x0007,0,0,1

   d) Up to 7 consecutive Modbus registers can be read simultaneously

In the body of the mapping table the 6th field is the number of consecutive fields to be read.  This can be increased up to 7, but both the Modbus register numbers and Sigma object numbers must be consecutive

31,2,3,0,1,7,0,0,1  ;ModBMS Test2 40001
38,2,3,7,1,4,0,0,1  ;ModBMS Test2 40008

3. Changes to the Sigma objects.  Note: these changes do not affect the Modbus update rate but can increase the displayed or graphic update rate.

     a) Scan priority number decrease

Selected Sigma objects can have there Scan Priority decreased or those Sigma objects that do not update as quickly increased allowing the IC3 Modbus controller to process the critical objects more often

     b) Tolerance value - particularly for Graphic, Dynaview and Actipane displays

Modify the Sigma object tolerance to match that of the object value.  See Lessons Learned Article #3264