8.2. Detailed Description of the SPM33 Modbus Protocol
8.2.1. SPM33 Modbus Protocol Rules
The following rules define the protocol rules for information transfer between a
MODBUS Master device and the SPM33 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
"PACKETS", a packet being simply a serial string of 8-bit bytes. The maximum
number of bytes contained within one packet is 255. 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.
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