Chapter 1: Product Introduction

1.1 Preface

This product is an economical MODBUS-RTU RS-485 relay module implemented with ASIC technology. The product may be powered from 5V, 9V, 12V, 18V, or 24V. This product supports all combinations of common baud rates, stop bits, and check bits, and its MODBUS-RTU address may be arbitrarily configured. These configurations are set through the RS-485 communication port, and through the button on the product, may be reset to the factory default, which is address 0x02, 9600 baud rate, 8 data bits, even parity, 1 stop bit. The functionality of the product is completely customizable in mass production as needed.

1.2 Structural Description

There are eight screw terminals, one button, and two LED indicators. The description of each terminal and component is shown in the table below. For B2J03, an extra white expansion header is present for the complimentary sensor analog input.

1.3 Wiring Methods

The power supply terminals are directly connected to the DC power input. The normally open and normally closed relay terminals are connected according to the application needs, and they are connected in series with the load and load power supply. The digital input of the product is connected to an external voltage, and its 485 communication port is connected to the 485 bus according to the corresponding polarity. For the transistor output type, the wiring diagram is described in section 4.2. A typical wiring diagram of the relay output type is shown below.

When the product lacks a case, it is only recommended for controlling the safe voltage (0-36V) rather than the AC mains. Special care should be taken to prevent electric shock when a caseless product is used to control the AC mains. Please be gentle when tightening the screws to avoid stressing the product PCB. Long-term reliability of the product may be affected when the screw is too overtightened. When the product has a case, it can be used to control the AC mains. It can optionally be glued onto a flat surface with double-sided adhesive tape.

Chapter 2: Detailed Function Description

2.1 Introduction to MODBUS-RTU

This product provides a half-duplex RS-485 serial communication port. It uses two serial differential lines to transmit data: A (negative polarity) and B (positive polarity). As RS-485 adopts differential signaling, the transmission distance is considerably far, reaching the order of several kilometers; if a repeater is added, the communications reach even further. The two lines of RS-485 are responsible for both receiving and sending, so they are half-duplex, that is, receiving and sending cannot be performed at the same time. For all modules on the same RS-485 bus, their serial communication parameters are all the same.

To communicate between the master and the module, a communication protocol called MODBUS-RTU is required to regulate the data transmission between them. MODBUS is an industrial standard field bus proposed by MODICON, which has become one of the first choices for industrial control. It includes MODBUS-RTU and MODBUS-ASCII, and makes use of multiple physical media. Specifically, this module uses the MODBUS-RTU protocol based on the RS-485 physical layer.

The data transmission of MODBUS-RTU is carried out in "frames". A "frame" is an Application Data Unit (ADU). In the ADU, when the address code header and the CRC16 checksum tail are removed, the remaining part is a Protocol Data Unit (PDU). The maximum ADU length supported by this module is 9 bytes, and ADUs exceeding this length will be directly truncated.

In the serial-based MODBUS-RTU, the default communication format used is 1 start bit, 8 data bits, 1 even parity bit, and 1 stop bit. The data is in Big Endian (BE), that is, the higher bytes get transmitted first. The "address code" in ADU is an 8-bit value; in order to communicate with the module normally, this value should be consistent with the module's setting. 0x00 is a special address used for broadcasting. This module responds to this address but does not reply any data. The final checksum is a 16-bit CRC16 checksum, and its generating polynomial is 0xA001. It is worth noting that the transmission of the CRC16 value is in Little Endian (LE), that is, the low byte is in front and the high byte is in the back. The CRC16 performs a check on the entire ADU, including the PDU and the address field.

All MODBUS-RTU function codes supported by this module are listed in the following sections, and unlisted commands are not supported. This module supports the circumstances where the CRC16 is not required. When reliability is not mandatory and development efficiency is at a premium, the CRC16 may be replaced with 0000, and this module also responds to such commands.

2.1.1 Read Coils

To read the value of the coil, use the "Read Coil" command (function code 0x01). The start address of the coils (two bytes) and the number of coils to be read (two bytes) need to be specified when reading the coils.

The maximum number of coils reads allowed by the MODBUS-RTU protocol itself is 0x07D0, while this module supports a maximum of 1. Therefore, the start address must be 0x0000, and the number of coils must be 0x0001.

In the normal return values, the coil data is packed into 8-bit bytes. The 8 lowest-address coils will be sent back first, followed by the 8 next lower address coils, and so on. In each byte, the highest bit corresponds to the highest-address coil. If the number of coils read back is not a multiple of 8, the missing high bits of the last byte will be filled with 0. If an error occurs during the execution of the instruction, an error message is returned.

In the example below, we assume that the slave address is 0xAA, and the master reads 11 coils from 0x0001-0x000B. Among these 11 coils, the odd address coils are all 1 (on), and the even address coils are all 0 (off).

2.1.2 Read Digital Inputs

To read the value of the digital input, use the "Read Digital Input" command (function code 0x02). The start address of the digital inputs (two bytes) and the number of digital inputs to be read (two bytes) need to be specified when reading the digital inputs.

The maximum number of digital input reads allowed by the MODBUS-RTU protocol itself is 0x07D0, while this module supports a maximum of 1. Therefore, the start address must be 0x0000, and the number of digital inputs must be 0x0001.

In the normal return values, the digital input data is packed into 8-bit bytes. The 8 lowest-address digital inputs will be sent back first, followed by the 8 next lower address digital inputs, and so on. In each byte, the highest bit corresponds to the highest-address digital input. If the number of digital inputs read back is not a multiple of 8, the missing high bits of the last byte will be filled with 0. If an error occurs during the execution of the instruction, an error message is returned.

In the example below, we assume that the slave address is 0xAA, and the master reads 13 input channels from 0x0000-0x000C. Among these 13 digital inputs, the odd address inputs are all 1 (high-level), and the even address inputs are all 0 (low-level).

2.1.3 Read Holding Registers

To read the value of the holding registers, use the "Read Holding Register" command (function code 0x03). The start address of the holding registers (two bytes) and the number of holding registers to be read (two bytes) need to be specified when reading the holding registers.

The maximum number of holding register reads allowed by the MODBUS-RTU protocol itself is 0x007D, while this module supports a maximum of 4. Therefore, the start address must not exceed 0x0003, and the number of registers plus the start address must not exceed 0x0004.

In the normal return value, the holding register data is returned in order. The lowest-address holding register will be sent back first, followed by the next lower address holding register, and so on. For each 16-bit value, the higher 8 bits are transmitted first, followed by the lower 8 bits. If an error occurs during the execution of the instruction, an error message is returned.

In the example below, we assume that the slave address is 0xAA, and the master reads 2 holding registers from 0x0000-0x0001. The value of the first register is 0x0BB8, and the value of the second register is 0x1B58.

2.1.4 Read Input Registers

To read the value of the input registers, use the "Read Input Register" command (function code 0x04). The start address of the input registers (two bytes) and the number of input registers to be read (two bytes) need to be specified when reading the input registers.

The maximum number of holding register reads allowed by the MODBUS-RTU protocol itself is 0x007D, while this module supports a maximum of 2. Therefore, the start address must not exceed 0x0001, and the number of registers plus the start address must not exceed 0x0002.

In the normal return value, the returned input data is returned in order. The lowest-address input register will be sent back first, followed by the next lower address input register, and so on. For each 16-bit value, the higher 8 bits are transmitted first, followed by the lower 8 bits. If an error occurs during the execution of the instruction, an error message is returned.

In the example below, we assume that the slave address is 0xAA, and the master reads 2 registers from 0x0000-0x0001. The value of the first register is 0x0BB8, and the value of the second register is 0x1B58.

2.1.5 Write Single Coil

To change the value of a single coil, use the "Write Single Coil" command (function code 0x05). The state constant in the request data field indicates the requested coil state: 0xFF00 requests the coil to turn on, and 0x0000 requests the coil to turn off. All other values are illegal and have no effect on the output. For the description of the coil of this machine, please refer to the "Read Coils" section.

In the normal return value, the coil address and status constant will be echoed. For each value, the higher 8 bits are transmitted first, followed by the lower 8 bits. If an error occurs during the execution of the instruction, an error message is returned.

In the example below, assume that the slave address is 0xAA, and the master turns on the coil at address 0x000B.

2.1.6 Write Single Holding Register

To write the value of a single holding register, use the "Write Single Holding Register" command (function code 0x06). The start address of the holding registers (two bytes) and the value of holding registers to be written (two bytes) need to be specified when writing the holding registers. For the description of the holding registers of this machine, please refer to the section "Read Holding Registers".

In the normal return value, the register address and register value will be echoed. For the register value, the upper 8 bits are transmitted first, followed by the lower 8 bits. If an error occurs during the execution of the instruction, an error message is returned.

In the example below, we assume that the slave address is 0xAA, and the master writes 0x0BB8 into the holding register at address 0x0001.

2.1.7 Possible Exception Codes

There is an exception code field in the error return value of MODBUS-RTU. The user can judge the problem that the device cannot execute the instruction correctly through the exception code. The four exception codes used in this machine are listed in the table below.

2.2 Serial Port Configuration Description

This product encompasses many combinations of baud rate, parity bit, and stop bit. These combinations are set through the serial port configuration holding register SRLCFG (address 0x0001). To protect SRLCFG from unintended modifications, a lock holding register LOCK (address 0x0002) is implemented to unlock the write to SRLCFG. A name holding register NAME (address 0x0000) is also implemented to facilitate users' identification of module instances, and writes to it also require the LOCK register to be unlocked. If a change to the name or configuration of the module is proposed, ASCII code "UL" (i.e., 0x554C) must be written to the lock holding register LOCK before writing NAME or SRLCFG. All writes to registers are effective immediately. For this particular module, only "Read Holding Register" (function code 0x03) and "Write Single Holding Register" commands (function code 0x06) are recognized for reading and writing these registers. The "Write Multiple Holding Registers" command (function code 0x10) is not supported.

To reset the module name and serial port configuration to factory default, all circuit connections to the module must be removed first. Keep the button pressed, then connect the power supply to the module. Release the reset button after 5 seconds and power cycle the module. The module then returns to the default settings of name ASCII code "UD" (i.e., 0x5544), address 0x02, 9600 baud rate, 8 data bits, 1 even parity bit, 1 stop bit. Further reconfiguration of the module may begin here.

It is certainly possible to use dedicated software to configure these parameters. When doing so, connect the module to the computer through a USB to RS-485 cable and specify the serial port number on the computer; the module may then be easily configured.

2.2.1 Lock Holding Register

The address of the lock holding register LOCK is 0x0002. To unlock the module configuration protection, please use the "Write Single Holding Register" command (function code 0x06) to write 0x554C to this address; writing any other value has no effect, and the value of the register will remain locked with a value of 0x0000. To check whether the unlocking is successful, use the "Read Holding Register" command (function code 0x03) to read LOCK and see if the value is 0x554C. After the register is successfully unlocked, the module will relock this register if it detects any instruction that targets other modules, instructions with parity errors, or instructions with wrong CRC16 on the bus. Reading this register will not cause re-locking. Writing to NAME and SRLCFG will cause relocking regardless of whether the write is successful or not.

2.2.2 Name Holding Register

The address of the name holding register NAME is 0x0000. This register is implemented to facilitate users' queries and module identification. To write to this register, the lock holding register LOCK needs to be unlocked. After unlocking, writing to this register will change the module name and relock the configuration lock.

2.2.3 Serial Port Configuration Holding Register

The address of the serial port configuration holding register SRLCFG is 0x0001. To write to this register, you need to unlock it as described in the previous section. After unlocking, writing to this register will configure the module parameters and relock the configuration lock. If an incorrect configuration is written to the register, the configuration will be rejected, the original configuration will remain unchanged, and the configuration lock will be relocked. The factory default configuration is 9600 baud rate, even parity, 1 stop bit, and MODBUS address 0x02.

2.2.4 Module Identification and Serial Port Testing

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Chapter 5: Electrical Specification Description

5.1 Normal Working Conditions

The normal working conditions of this module are shown in the table below. Only within this condition can the normal operation be guaranteed.

5.2 Absolute Maximum Ratings

The extreme conditions that this module can withstand are shown in the following table. Stresses at or above those listed here may permanently damage the module.

5.3 Other Parameters and Certifications

The other parameters and certifications of this module are listed as follows. These parameters and certifications include electrostatic discharge, electrical fast transient, and voltage surge tests that comply with the requirements of IEC61000-6-1-2016 (Edition 3.0, 2016-08) for light industry purposes. During all of these tests, the modules are powered by a well-grounded 12V supply and in active delay operation.

5.4 Notes on Reliability

This module is only qualified for commercial applications due to its miniature size, thus it is not recommended for formal industry situations with strong interference. Additional power filters and/or port filters are required if it must be used in this way. If such additional filters are not desired, a switching power supply with Electrical Fast Transient (EFT) resistance is required to power the module, and the power supply cord must be shorter than 3m.

In addition, if the load exhibits current spikes during startup or emits extremely strong EMI during operation, it is recommended to use standalone power sources for the module and to place these loads as far as possible from the module, or the module may repeatedly reset when the load powers on, especially in cases where the power source has poor transient responsiveness or the 5V version is used.

During the test, the negative supply of the module is directly connected to the earth by default. The same connection should be made to obtain the stability index in the table above.

Chapter 6: Appendix

6.1 Typical Application

A temporary LED lighting spotlight on a construction site needs to be remotely and automatically controlled by a computer. At the same time, there must be a switch beside the light, through which the light may be manually controlled. The LED spotlight is powered by a 12V battery.

6.1.1 Requirement Analysis

In this application, one input and one relay output are used, and the module connects to the computer through an RS-485 interface. The system is powered by a 12V supply and may be fulfilled with a module of model B2J031M31. Since there may be other modules mounted on the MODBUS-RTU bus, a unique communication address needs to be designated for this module. We assume that the address is 0x23. We also assume that the bus has a baud rate of 19200 and uses odd parity with 2 stop bits.