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Thursday, September 19, 2019
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t3.5 Between Transmit Frame in Modbus
How to use the t3.5 interval properly during master/slave communication


Could anyone help me to fully understand about modbus specification file?

1. In MODBUS over serial line specification and implementation guide V1.0 says:"A MODBUS message is placed by the transmitting device into a frame that has a known beginning and ending point. .... In RTU mode, message frames are separated by a silent interval of at least 3.5 character times."

This interval is used to detect frame endpoint between 2 sending frame by same one node, or it also be used to separate between one send frame and one receive frame between master/request and slave/response node?

in other word, between request and response frame, is the interval necessary?

2. Is the interval the only way to detect frame end, or the endpoint could be detected by receiving as long as the frame length completed, as specified frame all has fixed length, like similar way as ASCII mode? if count received length while receiving, once slave receives the expected length, it could response to master immediately without t3.5 expired. Is this a wrong usage?

earlier discussion involving the practicalities of the multi-vendor Modbus world shed some light on the topic:

Calculating the character gap

Lynn L's insight:

Delay between polls

Micrologix Master has override setting: