9.2. Detailed Description of SPM93 Modbus Protocol
9.2.1 SPM93 Modbus Protocol Rules
The following rules define the protocol for information transfer between a Modbus
Master device and the SPM93 in a RS-485 serial communications loop.
1) All communications on the RS-485 loop conforms to a Master/ Slave scheme. In
this scheme, information and data is transferred between a Modbus Master device
and up to 32 Slave monitoring devices.
2) The Master will initiate and control all information transfer on the RS-485
communications loop.
3) Under no circumstances will a Slave device initiate a communications sequence.
4) All communications activity on the RS-485 loop occurs in the form of "Packet", a
packet being simply a serial string of 8-bit bytes. The maximum number of bytes
contained within one packet is 128. The bytes that comprise a packet consist of
standard asynchronous serial data, which are generated using equipment similar to
that used for RS-232C.
5) The packages from Master are named request. The packages from Slave are
named response.
6) Under any circumstance, Slave can just respond one request.
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