The hardware detects the start-bit of the transmission and automatically enables (on the fly) the RS485 transmitter. A jumper labeled "1 485En" (J4) enables RS485 operation on the Serial1 port if the jumper cap is installed, and configures Serial1 for RS232 operation if the jumper cap is not installed. The BAUD routine described at the start of this subsection configures the baud rate of each of the Serial1 and Serial2 channels. InitRS485() configures Port D to ensure that bit 5 is an output. We start with including the standard library for driving the LCD and declare the D8 pin of the Arduino Nano as an output pin which we will later use to declare the MAX485 Module as a transmitter or Receiver. Here again, we start with including the standard library for driving the LCD and declare the D8 pin of the Arduino Nano as an output pin which we will later use to declare the MAX485 Module as a transmitter or Receiver. By setting this output LOW, rs485 cable the slave’s input /SS is pulled LOW. 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.

SPIE is a local interrupt mask that allows an interrupt to be recognized when an SPI data transfer has completed, or if a write collision or mode fault is detected. There are three flag bits implemented in the SPSR (SPI status register). So, for eight data bits with a parity bit, M would be set (equal to one) in order to add an extra bit to each byte transmitted, and PE would be set in order to make that extra bit be used as a parity bit. It is important to note that when the CPHA bit is 0, the /SS line must be de-asserted and re-asserted between each successive data byte exchange (68HC11 Reference Manual, Section 8.3.2). If the CPHA bit is 1, the /SS line may be tied low between successive transfers. The MODF bit is cleared by a read of the SPSR followed by a write to the SPCR. The end devices are responsible for terminating the cable so that there are no reflections from the cable ends.

Without termination resistors, reflections of fast driver edges can cause multiple data edges that can cause data corruption. The value of each termination resistor should be equal to the cable impedance (typically, 120 ohms for twisted pairs). Receiver Input Resistance (Ohms), (1 Standard Load for RS485) 3k to 7k 4k min. Two asynchronous communications ports named Serial1 and Serial2 can each be configured for RS232 or RS485 protocols. The RS232 signals are represented by voltage levels with respect to a system common (power / logic ground). To solve the "data collision" problem often present in multi-drop networks hardware units (converters, repeaters, micro-processor controls) can be constructed to remain in a receive mode until they are ready to transmit data. Systems of this type (4-wire, half-duplex) are often constructed to avoid "data collision" (bus contention) problems on a multi-drop network (more about solving this problem on a two-wire network in a moment). Also, RS485 drivers are able to withstand "data collisions" (bus contention) problems and bus fault conditions. They are often terminated with 8 pin RJ45 connectors at either end.

In other words, each local UART on the wildcard can both send data to and receive data from a remote UART on the other end of a connecting serial cable. The QScreen Controller has two serial communications ports: a primary serial port called Serial 1 that supports both RS232 and RS485 protocols, and a secondary serial port called Serial 2 that supports RS232. There are a variety of ways the MOSI, MISO, SCK and /SS pins on your QScreen Controller can be connected. We can gain insight into the operation of the RS232 protocol by examining the signal connections used for the primary serial port in Table 9 6. The transmit and receive data signals carry the messages being communicated between the QScreen Controller and the PC or terminal. RS232’s greatest benefit is its universality; practically all personal computers can use this protocol to send and receive serial data. If you have any questions, leave them in the comment section below or use our forums and I will try my best answering them. This page describes the serial ports and how to use them for instrument control and automation applications. Because the software UART is interrupt based, competing interrupts that prevent timely servicing of the Serial2 interrupts can cause communications errors on the secondary serial channel.