Modbus Protocols

Flavors of Modbus

RTU:

Common
Binary Protocol.
Active Master-passive Slave
Serial
Supported by FieldServers, QuickServers, CAS gateways

 

ASCII:

Similar to ModbusRTU but for each byte in an RTU message, there are 2 bytes in an ASCII message. The 2 bytes are the humand readable form of the single hex byte.
Eg RTU byte = 0x03 (Hex). ASCII bytes = ‘0’ and ‘3’ ie 0x30 and 0x33
Active Master-passive Slave
Serial
Supported by FieldServers, QuickServers

 

Jbus:

Modbus had the limitation of a max of 9999 items of each type. Ie only 9999 holding registers. However the protocol message allows 65k items to be addressed. JBUS allows all 65k items to be read/written. Other than that it is identical to RTU
Active Master-passive Slave
Serial
Supported by FieldServers, QuickServers, CAS gateways

 

TCP/IP:

Uses TCP/IP connection based Ethernet communications
Encapsulates RTU messages and adds a header.
A single slave can respond to multiple masters
Many slaves ignore the NodeID field in the message.
Supported by FieldServers, QuickServers, CAS gateways

 

MB Plus:

Proprietary coax networking layer
2 Mbits/sec
Supported by FieldServers, CAS gateways

 

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