The RS485 standard addresses the problem of data transmission, where a balanced (differential) transmission line is used in a multi-drop (party line) configuration (or point-to-point if only two devices are on the network). Since EIA-485 uses a voltage differential it can be used across large distances. Even though some IC manufacturers offer light loading devices, that can accommodate 256 or even 400 nodes on one RS485 network, you may NOT want to build such a network for a few reasons. It was developed for industrial applications, is relatively easy to deploy and maintain compared to other standards, and places few restrictions on the format of the data to be transmitted. While handshaking is still with us, it is usually best to disable it in software (if possible) and/or "loop-back" the pairs of signals (RTS to CTS, DTR to DSR, etc.) While RS232 was rumored to be on the "way out" with the advent of many of the new communications standards, it is still alive and well today. They are still both fairly widely used even though they are introduced more than half a century ago. There are a variety of ways the MOSI, MISO, SCK and /SS pins on your QVGA Controller can be connected.
There are surface mount resistor pads on the QScreen that will allow you to bring out the secondary serial port to the Field Header on pins 5-6 or 7-8 as shown with the parentheses in Table 11-3. Pads are also available to bring out the RS485 signals to the DB9 Serial 1 Connector. If you do this now, remember to move the QVGA Controller’s serial connector back to Serial Port 1, and to change the terminal’s baud rate back to 9600 baud using the "Communications" item under the terminal’s "Settings" menu. In multifunction instruments, not only the cables connected to the measurement, but also the RS485 serial port and the analog and digital inputs and outputs must be wired. If your application requires RS485, use the primary serial port (serial1) for RS485 communications, and use the secondary serial port (Serial 2) to program and debug your application code using the RS232 protocol. OSI and TCP/IP model (while Modbus is an application layer protocol). 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. MODBUS Application Protocol 2012, p.
For data encoding, Modbus uses a big-endian representation for addresses and data fields. The Modbus protocol uses serial communication lines, Ethernet, or the Internet protocol suite as a transport layer. These standard 9-pin serial connectors are located on the top edge of the QVGA Controller. Sig- lines are twisted as twisting nullify the effect of electromagnetic noise induced in a cable and provide a much better immunity against the noise which allows the RS485 to transmit the data up to 1200m of range. This way, the lines will be biased to known voltages and nodes will not interpret the noise from undriven lines as actual data; without biasing resistors, the data lines float in such a way that electrical noise sensitivity is greatest when all device stations are silent or unpowered. If there is no error during the execution process, the data field of the ADU response from server to client will include the data requested, i.e. the data the client previously received. In former terminology, the client was named master and the server named slave. With Modbus over Serial Line, client's role is implemented by master, and the server's role is implemented by slave. As long as there is no error during those steps, the server's responding function code will also be the function code sent from the client.
Thus, for a 16-bit value, the most significant byte is sent first. Function code is 1 byte which gives the code of the function to execute. Function code 01 (read coils) allow reading the state from 1 to 2000 coil of a remote device. 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. PLC systems are used to monitor and control operations, while HMI network systems allow interaction with machinery and control them. The software routines, what is rs485 cable RS232Transmit() and RS232Silent() control the dual RS232 transmitters on the board. This RS232 Shield is based on the MAX232, a dual driver/receiver with a capacitive voltage generator that can supply TIA/EIA-232-F voltage levels from a single 5-V supply. 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. A slave will never transmit any data or perform any action without a request from the master, and slaves cannot communicate with each other. 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.