HopNet RS-232 Serial ModBus Radio
command or reply then verifies the BCC or CRC and returns an ACK—if the BCC or
CRC is acceptable—or an NAK—if the
BCC or CRC does not check. The DF1 protocol also will indicate failure with an
error code. Internally, the link protocol must delimit messages, detect and signal
errors, retry after errors, and control message flow.
Modbus Operation
The Modbus protocol requires the gaps between bytes of data in the same packet or
message be within a specific limit.
Devices receiving the message then know the starting and ending point of a message.
These message frames are separated by a silent interval of at least 3.5 character times
and the entire message must be transmitted as a continuous stream. In cases where the
silent interval between bytes in a frame exceeds 1.5 characters, the frame is
considered incomplete and is discarded by the receiving device. Even though this is a
Modbus specification, not all Modbus devices strictly adhere to these timing
requirements.
When sending the Modbus packets wirelessly, messages can be split into two
transmissions from the sending radio. If one transmission must be re-transmitted, the
inter-character gap can be exceeded causing a Modbus error. To prevent this from
happening, the HopNet radios use an additional packetization scheme encapsulating
the Modbus packet. If Modbus mode is not selected, it is possible Modbus errors will
be experienced. Modbus mode is selected on the Parameter tab.
NOTE: When Modbus mode is selected it is no longer possible to change some of
the other radio timing parameters. The affected parameters will appear grayed out
on the Parameter tab.
The HopNet radio’s default data transmit delay is 45msec from the start of the packet,
then transmits and continues to stream the information to the remote. At slower baud
rates with longer data words, it is possible - during multiple re-tries – for a Modbus
timing error to be introduced. In such cases, the delay can be extended to allow the
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2000- 2004 Cirronet
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