With the Modbus RTU protocol, the start of a message begins with the first byte sent after a minimum silent interval (i.e. ~2 ms. at 19200 baud or ~3.6 ms. at 9600 baud). To strictly conform to the Modbus.org standard, the maximum time between sending any two successive bytes that belong to the same message should not exceed 1.5 character times (i.e. 780 us at 19200 or 1.56 ms at 9600 baud). However, the WattNode is more tolerant and allows a gap between bytes of nearly the minimum silent interval. Most serial port drivers today should not have a problem achieving this as long as the entire message is passed to the serial port driver in one call.

With many Modbus slaves you also need to be careful not to poll the slaves too quickly but the WattNode is more robust than most slaves are in this regard. Just be sure to wait at least 0.5 seconds before timing out on the reply (Note: On WattNode firmware version 16 or later, the reply timeout can be as short as 25 milliseconds). As a rule of thumb, many Modbus masters use a one second slave reply timeout to accommodate the slowest responding device on the bus since failing to allow enough time for a device to respond can result in bus contention.

The Modbus over Serial Line Specification & Implementation Guide V1.0 document from Modbus.org recommends much longer than this i.e. ‘from 1 s to several seconds at 9600 bps’.

See Also