Issue
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. 


Product Line
171CBU78090
171CBU98090
171CBU98091


Resolution
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.

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 20ms] / 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).