APAR AR602.B - RS485 COMMUNICATION INTERFACE (According to EIA RS-485); Modbus-Rtu Serial Transmission Protocol (Slave)

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11.3. RS485 COMMUNICATION INTERFACE (acc. To EIA RS-485)
The installation specification for RS485 interface is as follows:
Equipment from different manufacturers that form the RS485 network (e.g. RS485/USB converters) may have
integrated polarizing and terminating resistors; in such a case there is no need to use external elements.
When configuring the network, it is necessary to pay particular attention to the cabling installation
recommendations given in chapter 2.
11.4. MODBUS-RTU SERIAL TRANSMISSION PROTOCOL (SLAVE)
Baudrate and character format for RS485 and MODBUS-RTU address are set with parameters 74: r4br ,75: r4cF ,
76: Addr (chapter 8, Table 8, point VIII). Available functions: READ = 3 or 4, WRITE = 6.
Table 11.4.1. Query frame format for the READ function (frame length - 8 bytes):
address of
the device
function
4 or 3
register address to be
read: from Table 11.5
(chap. 11.5)
number of registers to be
read:
1 ÷ 13
CRC checksum
1 byte 1 byte 2 bytes (HB-LB) 2 bytes (HB-LB) 2 bytes (LB-HB)
Example 11.4.1. Reading of a register with address 0: 0x01 - 0x04 - 0x0000 - 0x0001 - 0x31CA
Table 11.4.2. Query frame format for the WRITE function (frame length - 8 bytes):
address of
the device
function 6
register address to be
written:
from Table 11.5 (chap. 11.5)
register value to be written CRC checksum
1 byte 1 byte 2 bytes (HB-LB) 2 bytes (HB-LB) 2 bytes (LB-HB)
Example 11.4.2. Writing a register with address 10 (0xA) with the value 0: 0x01 - 0x06 - 0x000A - 0x0000 - 0xA9C8
Table 11.4.3. Response frame format for the READ function (minimum frame length - 7 bytes):
address of
the device
function
4 or 3
number of bytes in the
data field (max. 13*2=26
bytes)
data field - register value CRC checksum
1 byte 1 byte 1 byte 2 ÷ 26 bytes (HB-LB) 2 bytes (LB-HB)
Example 11.4.3. Response frame for register value equal to 0: 0x01 - 0x04 - 0x02 - 0x0000 - 0xB930