Unbiased Report Exposes The Unanswered Questions on Rs485 Cable

We recommend that you keep the faster Serial1 port as the default serial link as you work through the exercises in this book. By polling the Port A pin or by setting up an interrupt service routine, you can configure the QScreen to ignore the SCK input when /SS is high and keep MISO in a high-impedance state so that it does not interfere with the SPI bus. Signal High and Lows are measured against the GND level so shifting the GND level will have a disastrous effect on the data transfer. Remember that the /SS is active low so to select a device you need to set the pin low; otherwise the pin should idle high. Rs485 is low impedance balanced , so twisting the pair helps with noise immunity as well as reducing the attenuation effect of higher baud rates due to the parallel capacitance of the wires in the cable jacket over the long lengths. 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. Since it uses a differential line over twisted pair (like EIA-422), it can span relatively large distances (up to 4000 feet or just over 1200 metres).

Most of the low-cost sensors and other modules like GPS, Bluetooth, RFID, ESP8266, etc. which are commonly used with Arduino, Raspberry Pi in the market uses UART TTL based communication because it only requires 2 wires TX(Transmitter) and RX (Receiver). Because they are produced in such large quantities, Cat 5 cables are relatively inexpensive, often less than half the price of specialty RS-485 cabling. In practice, Cat 5 cables have been used successfully in many installations, but there are some concerns. 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, 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. You need a special Ethernet cable for realizing this communication protocol. 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. An example of how a byte(0x3E) is transferred over the two lines of RS485 Communication. Shown below is the example of how a character is a transmitter over a UART data line.

For seven data bits with a parity bit, M would be cleared (equal to zero), and PE would be set in order to make the most-significant bit of a normal eight-bit byte be used by the serial port as a parity bit. RS232 uses inverse logic; that is, a positive bit at the 68HC11 UART is inverted by the onboard RS232 driver chip and appears as a negative signal on the serial cable. Rs485 cable is essentially a signal cable with a twisted pair structure, additionally shielded with 2 layers of noise protection used in industrial communication. This ensures maximum protection for the inner core. 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. Noise in the range of ±200 mV is essentially blocked due to common-mode noise cancellation. Termination resistors also reduce electrical noise sensitivity due to the lower impedance. The termination also includes pull up and pull down resistors to establish fail-safe bias for each data wire for the case when the lines are not being driven by any device. The pinout of the QScreen’s Serial Header (H5), QScreen’s Field Header (H3), and the Serial Connectors are shown in the following tables.

The default serial routines used by the onboard kernel assume that full duplex communications are available, so you cannot use the RS485 protocol to program the controller. Each of the two UARTs on the wildcard is capable of full-duplex communications, meaning that both transmission and reception can occur simultaneously (although the RS485 protocol is half duplex as explained below). The two lowest order bits in the SPCR control register, named SPR1 and SPR0, determine the data exchange frequency expressed in bits per second; this frequency is also known as the baud rate. For the transfer of data, the baud rates of both Master and Slave must be between 10% of each other. Polarity of the wires must be observed. Note that the local and the remote must share a common ground, so a minimum of 5 wires are required for full duplex RS422 communications: two transmit wires, two receive wires, and a common ground. The UART Wildcard provides two simultaneous communications links, each configurable as RS232, RS485 or RS422. Each packet contains 1 start bit, 5 to 9 data bits (depending on the UART), an optional parity bit, and 1 or 2 stop bits.