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How to calculate the ‘Delay Between Frames’ value for the inter-character gap time on a Unity Momentum serial Modbus port

  
Goals and Symptoms

The Modbus slave RS232 serial port of a legacy Momentum (Concept based program) does NOT adhere to the inter-character gap timing in RTU mode.  The Modbus slave RS232 serial port of a Unity Momentum (Unity based program) does adhere inter-character gap timing in RTU mode. 
 
Because of this difference, a Unity Momentum that replaces a legacy Momentum PLC used in radio/modem communications, may result in an increase in communication errors or limited operation. 

 
Facts and Changes
 
One solution may be to increase the Delay Between Frames, aka 'inter-character gap', timing of the slave port.  For example, to achieve an inter-character gap of 20ms, the Delay Between Frames value must be changed from its default value of 4 to 47.  This does not mean the delay was increased from 4ms to 47ms.  It is a value that results in a specific inter-character gap time calculation.


Causes and Fixes

To determine the value required for a specific inter-character gap time, the following equation is used.  The example below shows how a value of 47 was calculated for a 20ms inter-character gap requirement. 
 
(Delay Between Frames value = (7 x intercharacter gap required in ms) / 3)

  Delay Between Frames value = [7 x 20] / 3)
  Delay Between Frames value = [140 / 3)
  Delay Between Frames value = 46.6  (47)

The result is then entered into the 'Delay between frames' field (as indicated below).
 
 




 
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