The secondary serial port is connected similarly except that the onboard connection of RTS to CTS, and DSR to DTR are permanent. It also has onboard LEDs to display the current state of the chip i.e. whether the chip is powered or its transmitting or receiving data making it easier to debug and use. The driver is limited for short-circuit current and the driver outputs can be placed at a high impedance state through the thermal shutdown circuit. The capacitance is close to 12.5 pF/foot but there is a small impedance mismatch (100 ohms for Cat 5, 120 ohms for RS-485). This UART-RS485 Converter module has an on-board MAX485 chip which is a low-power and slew-rate-limited transceiver used for RS-485 communication. In fact, a single driver chip on the UART Wildcard is used to implement both RS422 and RS485 communications for a given serial channel. They should generally not be needed, except if you use long cables, multiple RS485 devices, and resistive termination. After configuring the SPI system to communicate on a properly connected network of devices, sending and receiving data is as simple as writing and reading a register.

There are many different types of serial communication protocols like I2C and SPI which can be easily implemented with Arduino and today we are going to look at another most commonly used protocol called RS485 which is very commonly used in high noise industrial environments to transfer the data over a long distance. If you are using the QScreen as a master device, each external SPI device will require a separate select line (/SS). When the /SS input goes low, the slave (or QScreen in this case) transfers data in response to the SCK clock input that is initiated by the master. The clock’s polarity is controlled by a bit named CPOL (clock polarity) and its phase is controlled by CPHA (clock phase). In some cases, however, a sophisticated network may have device groups on a network that use different clock configurations. These detailed signal descriptions and cable diagrams are presented to provide complete information for those who have special communications requirements and for those who wish to make their own application-specific communications cables. The RS485 receiver compares the voltage difference between both lines, instead of the absolute voltage level on a signal line. The serial output at the UART idles at the logic high (mark) level.

The receiver input has a fail-safe feature that guarantees logic high output if the input is open circuit. Typical line voltage levels from the line drivers are a minimum of ±1.5 V to a maximum of about ±6 V. Receiver input sensitivity is ±200 mV. 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. 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. They have Shielding Jacket over the insulation layer to protect against the Electromagnetic Interference and also each pair of wires is twisted together to prevent any current loop formation and thus much better protection against the noise.

The main reason behind using these Ethernet Cables over normal wires is that they provide much better protection against noise creeping in and distortion of the signal over high distances. RS-485, like RS-422, can be made full-duplex by using four wires. There are many categories of Ethernet cables we can use like CAT-4, CAT-5, CAT-5E, CAT-6, CAT-6A, etc. In our tutorial, rs485 cable we are going to use CAT-6E cable which has 4 twisted pairs of 24AWG wires and can support up to 600MHz. It is terminated at both ends by an RJ45 connector. In this tutorial, we are going to learn about the RS485 communication protocol and how to implement it with the two Arduino Nano we have with us and how to use the MAX485 RS485 to UART conversion Module. The PLC is also connected to the VFD via RS485. We will send some data from the transmitter side over the cable from the Nano which is converted to RS485 signals via MAX RS485 Module working in Master Mode. An example of how a byte(0x3E) is transferred over the two lines of RS485 Communication.