In this mode, a Modbus transaction includes two messages: one request from the master and one reply from the slave. Function Code: Function code's MSB bit of Exception is 1. This will make this function code 0x80 higher than then request message function code. 2 wires is all you need to make it work. Make sure the SIM has the PIN disabled. If you decide to buy non-FTDI cables, then you have to ensure they work on linux or a Raspberry PI without installation of extra drivers and software. Server will then read/parse the function code, get the address of the data field of the PDU, then get this data field value and finally perform the action based on the function code. The data field of the PDU can be empty, and then has a size of 0. In this case, the server will not request any information and the function code defines the function to be executed. Modbus defines a protocol data unit (PDU) independently to its lower layer protocols in its protocol stack.

Modbus is often used to connect a plant/system supervisory computer with a remote terminal unit (RTU) in supervisory control and data acquisition (SCADA) systems. 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. RTU (Remote Terminal Unit) mode transmits 8-bit messages as two 4-bit hexadecimal characters in a continuous stream. The mapping of MODBUS protocol on specific buses or network requires some additional fields, which are defined as application data unit (ADU). Modbus defines client which is an entity which initiates a transaction to request any specific task from its "request receiver". Modbus defines three types of function codes: Public, User-Defined and Reserved. Modbus gives two range of values for user-defined function codes: 65 to 72 and 100 to 110. Obviously, user-defined function codes are not unique. Reserved Function Codes are function codes used by some companies for legacy product and are not available for public use. Function codes are integer values, ranging from 1 to 255, and the range from 128 to 255 is for exception responses. Function code 01 (read coils) allow reading the state from 1 to 2000 coil of a remote device.

Function code is 1 byte which gives the code of the function to execute. Data: Server returns the exception code inside the Data field. Many of the data types are named from industrial control of factory devices, such as ladder logic because of its use in driving relays: a single-bit physical output is called a coil, and a single-bit physical input is called a discrete input or a contact. Contact us, to get additional information. The master doesn't need to have an address. A serial bus for Modbus over Serial Line can have a maximum of 247 slaves communicating with 1 master. And even worse, if you have a non opto-isolated device installed on that network, any noise could "drain" to ground through the device itself instead of flowing through the ground terminal at the end of the line, damaging the device in the process! Those slaves have a unique address ranging from 1 to 247 (decimal value). Modbus Serial Line protocol is a master-slave protocol which supports one master and multiple slaves in the serial bus. A slave will never transmit any data or perform any action without a request from the master, and slaves cannot communicate with each other.

The communication process is initiated by the master, as only it can initiate a Modbus transaction. 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. Modbus standard also defines Modbus over Serial Line, a protocol over the data link layer of the OSI model for the Modbus application layer protocol to be communicated over a serial bus. RS485 is a serial data transmission interface standard that enables communication with sensors attached to the same bus (wires). LAPP manufactures a range of Unitronic Serial RS485 Cable range. Up to 10 WIRED PROs can be connected to SENSEWAY within a total range of 100m of RS485 cable. AIR Commander Entire includes standard micro-USB cable for camera connection however, the Marshall connection needs special RS485 adapter cable. The most common cable type is Cat 5e-UTP (unshielded twisted pair) which may work over shorter distances in less demanding applications with low EMI noise levels.