Athena C10 line Manual de Instruções descarregar pdf (Página 6)

1. MODBUS Protocol

The Modbus protocol defines the format and method of communications

between a “master” which controls the system and one or more “slaves”

which respond to commands sent by the master. The Modbus defines

how the transmitter and reciever are identified, how messages are

exchanged and how errors are detected.

There may be one master and up to 247 slaves on a common line; this is

the protocol’s logical limit, the physical interface may limit the number of

devices further, for example, the standard RS485 interface handles a

maximum of 31 slaves connected on the line. Substituting the last device

on the line with a proper “repeater” or “bridge”, another 31 instruments

can be added to the line and so on up to the above mentioned limit (247).

All transactions are started by the master. A transaction may be a direct

question/reply to a single slave or broadcast in which the message is

sent to all the devices on the line and no answer is given. A transaction

consists of a single question/replay frame or a single broadcast

message/no answer frame.

Some of the characteristics of the protocol are not defined. These are:

standard interface, baud rate parity, number of bits.

The protocol also enables the user to choose between two

communications modes, ASCII and RTU (binary). Only the RTU mode is

implemented on ATHENA CONTROLS instruments as it is more efficient.

The Jbus protocol is functionally identical to Modbus and differs from it in

how the addresses are numbered: with Modbus the addresses are

numbered starting from zero) 0000= 1

address, while with Jbus they

start from one (0001=1

address). This difference is maintained

throughout all addresses. From here on, unless explicitly specified, even

though reference is made to Modbus, the description is valid for both.

2. Message Format

For communication to take place between the two devices, the message

must be put in a "packet". The packet leaves the transmitter through a

"port" and is "carried" along the line to a similar "port" on the receiver.

MODBUS establishes the format of this packet which includes, for both

the master and the slave:

• The address of the device with which the master has established the

connection (address 0 corresponds to a broadcast message sent to

all slave devices).

• The code of the function that is to be or has been performed.

• The data that is to be exchanged.

• The error check based on the CRC16 algorithm.

If a device detects an error in the message received (in the format, parity

or CRC16) the message is considered invalid and rejected, a slave that

detects an error in the message will therefore not take any action or

answer the question, such as when the address does not correspond to a

device on the line.

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Athena C10 line Manual de Instruções Página 6

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