APAR AR553, User Instruction

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12. RS485 COMMUNICATION INTERFACE (acc. to EIA RS-485)
The installation specification for the RS485- standard interface is the following:
- maximum cable length - 1 km (observe the installation guidelines, chapter 2, sub-items b, c, and d)
- maximum number of devices in a RS485 line - 30, in order to increase the number, use RS485/RS/485 amplifiers
- termination and polarizing resistors when the MASTER is at the start of the line (Fig. 12):
- at the start of the line - 2 x 820 Ω to the ground and +5 V of the MASTER and 150 Ω between lines
- at the end of the line - 150 Ω between lines
- termination and polarizing resistors when the MASTER is in the center of the line:
- at the converter - 2 x 820 Ω, to the ground and +5 V of the converter
- at both ends of the line - 150 Ω each between lines
Equipment from different manufacturers that form the RS485 network (e.g. RS485 converters/USB) may have
integrated polarizing and terminating resistors; in such a case there is no need to use external elements.
Fig. 12. Pictorial diagram of the RS485 network
13. MODBUS–RTU SERIAL TRANSMISSION PROTOCOL (SLAVE)
Character format : 8 bits, 1 stop bit, no parity bit
Available functions : READ - 3 or 4, WRITE - 6
Table 13.1. Query frame format for the READ function (frame length - 8 bytes):
address of
the device function
4 or 3 read register address:
0 ÷ 3
number of read registers:
1 ÷ 4
CRC check sum
1 byte 1 byte 2 bytes (HB-LB) 2 bytes (HB-LB) 2 bytes (LB-HB)
Example 13.1. Reading of a register with address 0: 0x01 - 0x04 - 0x0000 - 0x0001 - 0x31CA
Table 13.2. Query frame format for the WRITE function (frame length - 8 bytes):
address of
the device
function 6
write register address:
0 ÷ 3
write register value CRC check sum
1 byte 1 byte 2 bytes (HB-LB) 2 bytes (HB-LB) 2 bytes (LB-HB)
Example 13.2. Entry in a register with address 10 (0xA) with the 0 value: 0x01 - 0x06 - 0x000A - 0x0000 - 0xA9C8