The serial links support two communication protocols:
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Modbus Serial
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Character Mode
Modbus is a standard with the following properties:
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Establishes client/server communication between different modules within a bus or serial link. The client is identified by the master and the slave modules represent the servers.
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Is based on a mode of data exchange composed of requests and responses offering services via different function codes.
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Establishes a means of exchanging frames from Modbus-type applications in two types of code:
The exchange management procedure is as follows:
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Only one device may send data on the bus.
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Exchanges are managed by the master. Only the master may initiate exchanges. Slaves may not send messages without first being invited to do so.
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In the event of an invalid exchange, the master repeats the request. The slave to which the request is made is declared absent by the master if it does not respond within a given time scale.
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If the slave does not understand or cannot process the request, it sends an exception response to the master. In this case, the master may or may not repeat the request.
Two types of dialogue are possible between master and slave(s):
Character Mode Communication
Character mode is a point-to-point mode of data exchange between two entities. Unlike Modbus Protocol, it does not establish hierarchically structured serial link communications or offer services via function codes.
Character Mode is asynchronous. Each item of textual information is sent or received character by character at irregular time intervals. The time taken by the exchanges can be determined from the following properties:
