RS485 Modbus Relay PRO (64, 12V, CN, None, A)

Overview

This device communicates over an RS‑485 half‑duplex bus using the Modbus RTU protocol. It supports common function codes for reading/writing coils and holding registers, and provides configuration for channel work modes, batch switching, timing/pulse functions, etc.

Wiring Instructions

- Connect the A terminal to the master/gateway's A (DI+). - Connect the B terminal to the master/gateway's B (DI-). - If there are many devices on the bus, it is recommended to add a 120 Ω termination resistor at the end of the bus. - When multiple devices are paralleled, keep the topology bus‑type as much as possible, avoid star topology. - Ground the shielding at one end only, and use twisted‑pair cables for communication lines.

Common Function Codes

Function Code	Function	Address Range	Bit/Word Operation	Quantity
01	Read Coil Status	0000H–0040H	Bit operation	Single or multiple
03	Read Holding Registers	0000H–008FH	Word operation	Single or multiple
05	Write Single Coil	0000H–0040H	Bit operation	Single
06	Write Single Holding Register	0000H–008FH	Word operation	Single
0F	Write Multiple Coils	0000H–0040H	Bit operation	Multiple
10	Write Multiple Holding Registers	0000H–008FH	Word operation	Multiple

Communication Parameters

- Baud rate: default 38400, configurable (see register 0043H). Supported examples: 4800/9600/14400/19200/38400/56000/57600/115200. - Data bits: 8 - Parity: configurable (see register 0044H), 0=None, 1=Odd, 2=Even. - Stop bits: typically 2 when no parity, typically 1 with odd/even parity (depends on master and field). - Slave address range: 1–255 (see register 0042H).

Coil and Register Addresses

Coils (Coils, bit operation, function codes 01/05/0F)

- Range: 0000H–0040H (example). - 0000H…0007H: corresponds to relay channels 1…8. - Higher channels follow sequentially; for batch bit writes, the LSB corresponds to the starting address.

Holding Registers (Holding Registers, word operation, function codes 03 read / 06 write single / 10 write multiple)

Address (HEX)	Name / Function	Value / Description
0000H	Channel 1 control register (work mode and action)	1) work_mode_normal: Normal mode, 1/0 = ON/OFF; 2) work_mode_flip: Toggle (flip) mode, "write 1 to toggle ON/OFF"; 3) work_mode_delay: Delay mode, delay N×0.01 s per operation; 4) work_mode_loop: Switch loop mode, period/count determined by holding time (see 0040H/0041H); 5) work_mode_auto_reset: Auto‑reset mode, automatically turns OFF after N×0.01 s, retains state after power loss. (The actual enumeration follows the definitions in channel mode registers starting at 0050H.)
003FH	Channel 64 control register	same as above (channel index increments)
0040H	Communication watchdog time setting	Watchdog time N × 0.1 s. 0: no watchdog; >1: when communication is lost, after N × 0.1 s all outputs are turned off; state is retained after power loss and takes effect after power‑on.
0041H	Input report / action count setting	0: no proactive reporting; 1: report immediately when any input window state changes; >1: accumulate and report/action after (N‑1)×0.01 s; retains after power loss, effective after power‑on.
0042H	Slave address	1–255; example read = 1; retained after power loss, effective after restart.
0043H	RS‑485 baud rate	0:4800, 1:9600, 2:14400, 3:19200, 4:38400, 5:56000, 6:57600, 7:115200; retained after power loss, effective after restart.
0044H	Parity	0: None, 1: Odd, 2: Even; retained after power loss, effective after restart.
0045H	Relay batch control	1: all ON; 0: all OFF.
0046H	Bit control channels 1–16	bit=1 output ON, bit=0 OFF.
0046H	Bit control channels 17–32	bit=1 output ON, bit=0 OFF.
0048H	Bit control channels 33–48	bit=1 output ON, bit=0 OFF.
0049H	Bit control channels 49–64	bit=1 output ON, bit=0 OFF.
004AH	Software reset	Write 1 to execute reset; after reset this bit automatically returns to 0.
004BH–004FH	Reserved	—
0050H	Channel 1 work mode	0: normal; 1: toggle; 2: timed; 3: switch loop; 4: auto reset.
…	…	…
008FH	Channel 64 work mode	same as above, 0: normal; 1: toggle; 2: timed; 3: switch loop; 4: auto reset.

Frame Format

- Frame format: Address (1 B) + Function Code (1 B) + Data (N B) + CRC16 (low byte first, high byte second). - Inter‑frame interval: ≥3.5 character times (RTU specification). - CRC: polynomial 0xA001, initial value 0xFFFF (see code below).

CRC Reference Implementation (C)

uint16_t factory_crc16(uint8_t* bufData, uint16_t buflen)
{
    uint16_t TCPCRC = 0xFFFF;
    uint16_t POLYNOMIAL = 0xA001;
    uint8_t i, j;

    for (i = 0; i < buflen; i++)
    {
        TCPCRC ^= bufData[i];
        for (j = 0; j < 8; j++)
        {
            if ((TCPCRC & 0x0001) != 0)
            {
                TCPCRC >>= 1;
                TCPCRC ^= POLYNOMIAL;
            }
            else
            {
                TCPCRC >>= 1;
            }
        }
    }
    return TCPCRC;
}

Online CRC Calculation

23bei Online CRC Calculator

Communication Example Frames

01: Read Coil Status (Master Request)

Read start address 000AH, quantity 000AH (example)

01 01 00 0A 00 0A 9C 0F

01: Read Coil Status (Slave Response)

Returns 2‑byte bitmap: 000AH–0011H, 0012H–0019H (example)

01 01 02 F0 03 BD FD

03: Read Holding Registers (Master Request)

Read 2 registers starting from 0002H (example)

01 03 00 02 00 02 C4 0B

03: Read Holding Registers (Slave Response)

Returns 4‑byte data: registers 0000H/0001H (example values)

01 03 04 F0 C3 A0 4F 00 FB

05: Write Single Coil (Master Request / Slave Echo)

Set coil 000AH ON (FF00 = ON)

01 05 00 0A FF 00 AC 38
01 05 00 0A FF 00 AC 38

06: Write Single Register (Master Request / Slave Echo)

Write register 0013H = 0x15C2 (example)

01 06 00 13 15 C2 F7 0E
01 06 00 13 15 C2 F7 0E

0F: Write Multiple Coils (Master Request / Slave Response)

Write 10 coils starting at 000AH, total 2 bytes (example)

01 0F 00 0A 00 0A 02 AA 0A 1A F3
01 0F 00 0A 00 0A 1A F3

10: Write Multiple Holding Registers (Master Request / Slave Response)

Write 2 registers starting at 0013H, total 4 bytes (example values 0x1253, 0x3C1F)

01 10 00 13 00 02 04 12 53 3C 1F 17 17
01 10 00 13 00 02 B0 0D

Batch Bit Control

- 0046H–004FH: control channels 1–16 (bit=1 turns output ON). - 0048H: control 33–48; 0049H: control 49–64. - 0045H: batch all‑ON/all‑OFF (1=ON, 0=OFF).

Channel Work Mode Settings

- Starting at 0050H, each channel occupies 1 word; example enumeration:

 - 0: normal mode (direct ON/OFF).
 - 1: toggle mode (write 1 to toggle).
 - 2: timed mode (configured with 0040H/0041H).
 - 3: switch loop mode.
 - 4: auto‑reset mode (automatically turns OFF after timeout).

- 008FH: channel 64 work mode.

Device Management and System Registers

- 0042H: slave address (1–255), retained after power loss, effective after restart.

- 0043H: baud rate (enumeration as above), retained after power loss, effective after restart.

- 0044H: parity (0/1/2), retained after power loss, effective after restart.

- 004AH: software reset (write 1 to execute, bit returns to 0 after reset).

Common Troubleshooting

- No response:

 - Verify that slave address, baud rate, and parity match the master.
 - Check that A/B lines are not reversed; ensure termination resistor on the bus.
 - Increase master timeout and retry count.

- CRC error:

 - Ensure CRC low byte is first; confirm no extra spaces/separators in the frame.
 - Try lowering baud rate, check shielding and grounding.

- Intermittent frame loss:

 - Extend inter‑frame interval, optimize transmit/receive switching timing.
 - Investigate power supply and common‑ground issues.

Related Downloads

1. DIUSTOU serial Modbus relay debugging software

  This software requires a USB‑to‑RS485 module.

File:Modbus relay a.zip

2. pymodbus example files, requires Python 3.7+ and the pymodbus package:

  pip install pymodbus

File:Modbus relay 485.zip