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The local and remote must share a common ground, so all serial cables include at least one ground conductor. 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 equipment located along a set of RS-485 wires are interchangeably called nodes, stations or devices. In the most common multi-drop RS485 protocol, one computer is designated as a master and the rest of the computers or devices on the serial bus are designated as slaves. With careful design, many peripherals can communicate via the SPI, and powerful multi-processor systems can be linked using this high speed bus. RS485Receive() to wait for any pending character transmission to complete, then disable the transmitter, and then execute a routine such as Key() to listen to the communications on the serial bus. The UART Wildcard provides two simultaneous communications links, each configurable as RS232, RS485 or RS422. 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.

This is an extra single bit appended to the end of each byte or character transmitted, which is set or cleared as necessary to ensure that the total number of '1' bits in the byte is always odd or even. One to two logic-high stop bits mark the end of a character. 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 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. This allows for basic error detection, in that if noise on the transmission line causes one bit to be received incorrectly, either received as a '0' when transmitted as a '1' or vice-versa, the error would be detected due to the count of '1' bits in the byte being odd when it is expected to be even, or vice-versa depending on the parity checking settings. If a slave device has already stored a byte into its SPDR register, that byte will be exchanged with the master’s byte.

If your application requires communicating with a device that expects to receive a parity bit, the generation of a parity bit and selection of even or odd parity, and whether there are seven or eight data bits in each byte, is performed by setting or clearing bits in the configuration registers SCI0CR1 for Serial1 and SCI1CR1 for Serial2. To ensure that no two devices drive the network at the same time, it is necessary that each slave device be able to disable its own RS485 data transmitter. The A signals of all devices should be connected with each other and the same goes for the B signals. To interface devices that support synchronized serial interfaces, but are not configurable like the QScreen, determine the device’s requirements for clock phase and polarity and configure the QScreen’s CPHA and CPOL accordingly. You may use nonstandard baud rates if both devices support them. Software-selectable baud rates up to 56,000 baud are supported.

Baud rates up to 56,000 baud are supported. Modem to phone line communications use rates of 110, 150, 300, 600, 1200, 2400, 3000, and 3200 baud. There are different sets of standard baud rates in use depending on the application. The standard C serial I/O routines such as printf(), scanf(), putchar(), and getchar() give you high level access to the serial ports. 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. Rather, the transmitter and receiver must be communicating using a known baud rate, or bit frequency. So long as the error between the actual baud rate and that specified is less than 1.5% (or the error between transmitter and receiver is less than 3%) there should be no communication errors. Rs485 cable is essentially a signal cable with a twisted pair structure, additionally shielded with 2 layers of noise protection used in industrial communication. In this example, the QScreen Controller selects the serial A/D by outputting a LOW signal on /SS.