Athena C10 line Manual de Instruções descarregar pdf (Página 7)

2.1 Characters Format

The above mentioned packets referred to the transmitted character from

the PC or Supervisor.In this case this character cannot be modified

because they are set by Athena Controls. The default value is 8 , N, 1. It

means 8 data bits without parity check and with 1 stop bit.

2.2 The Address

As mentioned above, MODBUS transactions always involve the master,

which handles the line, and one slave at a time (except in the case of

broadcast messages). To identify the user to whom the message is sent,

the first character sent is a byte containing the numeric address of the

selected slave device. Each of the slaves will therefore be assigned a

different address that identifies it uniquely. The valid addresses range

from 1 to 247, while address 0, which cannot be assigned to a slave, set

at the start of the message sent by the master indicates that the message

is to be "broadcast", that is, sent to all the slaves at the same time.

Broadcast messages are exclusively those that do not require an answer

to carry out their function, i.e. assignments only.

2.3 The Function Code

The second character in the message identifies the function that is to be

performed in the message sent by the master, to which the slave

answers by sending back the same code to indicate that the function has

been performed. On ATHENA CONTROLS instruments, a subset of the

MODBUS functions has been implemented as follows:

• 01 Read Coil Status

• 02 Read Input Status

• 03 Read Holding Registers

• 04 Read Input registers

• 05 Force Single Coil

• 06 Preset Single register

• 07 Read Status

• 15 Force Multiple Coils

• 16 Preset Multiple Registers

In the implementation for ATHENA CONTROLS instruments, functions 01

and 02 are functionally identical and interchangeable, as are functions 03

and 04. For a full and detailed description of the functions, see chapter 3.

2.4 The CRC 16 Algorithm

The last two characters in the message contain the Cyclic Redundancy

Check based on the CRC16 algorithm. To calculate these two

characters, the message (address, function code and data without the

start, stop and parity bits) is considered as a single continuous binary

number whose most significant bit (MSB) is sent first. The message is

first multiplied by x

(shifted to the left by 16 bits) and then divided by

+1 expressed as a binary number (1100000000000101). The

integer part of the quotient is then rejected and the 16 bit remainder

(initialised at FFFFh at the start to avoid messages consisting exclusively

of zeros) is added on to the end of the message sent. The resulting

message, when divided by the same polynomial (2

+1) by the

receiving device must give zero as a remainder if no errors occurred (the

receiving device recalculates the CRC).

1 2 3 4 5 6 7 8 9 10 11 12 ... 106 107

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Athena C10 line Manual de Instruções Página 7

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