[ModBUS] Serial modbusRTU timeout happens randomly for no visible reason

There's been observed a serial modbusRTU "timeout" error with both
writes and reads. Both errors are intermittent, but here's a error case recorded with a logic analyzer.

• Please see Capture1_AppOutput for the error as reported in that application
  output. Notice the error after the first RTU write.
• Please see Capture1_AllThreeMsgs for a view of all three writes. Notice the
  packet data of the first request matches the data from the first write of the
  application output.
• Please see Capture1_TimedoutMsg to see the timing of the slave's response
  from that first write request. After 53msec a proper formed response was sent
  by the slave. Yet this produced an error with timeout set to 2000msecs.

Retries set to 0. All other internal settings are at default.