With Modbus over Serial Line, client's role is implemented by master, and the server's role is implemented by slave. The mapping of PDU of Modbus to the serial bus of Modbus over Serial Line protocol results in Modbus Serial Line PDU. Differences between Modbus and JBUS at that time (number of entities, server stations) are now irrelevant as this protocol almost disappeared with the April PLC series, which AEG Schneider Automation bought in 1994 and then made obsolete. MODBUS over Serial Line has two transmission modes RTU and ASCII which are corresponded to two versions of the protocol, known as Modbus RTU and Modbus ASCII. Modbus RTU (Remote Terminal Unit), which is the most common implementation available for Modbus, makes use of a compact, binary representation of the data for protocol communication. Each slave must have a unique address (from 1 to 247) to be addressed independently for the communication. Modbus over UDP - some have experimented with using Modbus over UDP on IP networks, which removes the overhead of TCP.

Modbus over TCP/IP, Modbus over TCP, or Modbus RTU/IP - a variant that differs from Modbus TCP in that a checksum is included in the payload, as with Modbus RTU. Unit identifier is used with Modbus TCP devices that are composites of several Modbus devices, e.g. Modbus TCP to Modbus RTU gateways. To make sure that your USB to RS-485 cable & the device RS-485 port are working, have you tried using them with a Windows PC and the manufacturer's s/w? Those slaves have a unique address ranging from 1 to 247 (decimal value). A slave will never transmit any data or perform any action without a request from the master, and slaves cannot communicate with each other. In unicast mode, the master will initiate a request to a single slave with a specific address. The communication process is initiated by the master, as only it can initiate a Modbus transaction. TIA/EIA-232-E (RS232) can also be used but is limited to point-to-point short-range communication. One transmitter and 10 receivers are allowed on a network, with a distance limit of 3600ft. RS422 was on the market prior to RS485; however, due to loading limitations, one of the best uses of RS422 is probably in point-to-point communications, such as RS232 extension cords.

There are three flag bits implemented in the SPSR (SPI status register). Finally, for master devices, the SPR1 and SPR0 bits determine the baud rate at which data is exchanged. Like RS232, the data bits are transmitted in reverse order, with the least significant bit transmitted first. Modbus ASCII messages are framed by a leading colon (":") and trailing newline (CR/LF). The byte-sized messages are transmitted and received via the MOSI (master out/slave in) and MISO (master in/slave out) pins. Modbus messages are framed (separated) by idle (silent) periods. Data types are mapped using standard addresses. Data models and function calls are identical for the first four variants listed above; only the encapsulation is different. Address, Function, Data, and LRC are ASCII hexadecimal encoded values, whereby 8-bit values (0-255) are encoded as two human-readable ASCII characters from the ranges 0-9 and A-F. LRC is calculated as the sum of 8-bit values (excluding the start and end characters), negated (two's complement) and encoded as an 8-bit value. For higher data rates, Modbus RTU recommends to use the fixed values 750 μs for t1.5 and 1.750 ms for t3.5.

A Modbus RTU message must be transmitted continuously without inter-character hesitations. Upon receiving and finishing the request, the slave will respond with a message to the master. If a slave device has already stored a byte into its SPDR register, that byte will be exchanged with the master’s byte. A write collision occurs when a byte is written to the SPI data register, SPDR, what is rs485 cable while data is being exchanged. 0x00 and 0x01 : high byte and low byte of the number of registers to read. 0x00 and 0x01 : high byte and low byte of the register address to read. Example of a Modbus RTU frame in hexadecimal: 01 04 02 FF FF B8 80 (CRC-16-MODBUS calculation for the 5 bytes from 01 to FF gives 80B8, which is transmitted least significant byte first). The RTU format follows the commands/data with a cyclic redundancy check checksum as an error check mechanism to ensure the reliability of data. The Error check field with CRC/LRC: The error check methods depend on the protocol versions of the MODBUS over Serial Line, whether it is Modbus RTU or Modbus ASCII.