25
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
