The end resistance must be used only at the ends of the main cable. In order to avoid signal reflections, a 120 Ohm termination resistance must be fitted on each end of the main cable. The local and remote must share a common ground, so all serial cables include at least one ground conductor. RS485 avoids this issue which also allows for shielding of the cables to prevent noise. Access control systems: Rs485 cables are mainly used to connect and transmit data between access control devices such as security gates, RFID cards, or time attendance machines and the control center. This section describes the driver routines that control the RS485 transceiver, and presents some ideas that may prove useful in designing a multi-drop data exchange protocol. This section describes the QED-Forth routines that control the RS485 transceiver, and presents some ideas that may prove useful in designing a multi-drop data exchange protocol. Be sure to account for these effects when designing your application. To avoid contention on the RS485 bus, the application software must assure that only one transmitter is enabled at a time. Rather, the transmitter and receiver must be communicating using a known baud rate, or bit frequency.

The RS232 driver and receiver use separate conductors on the serial cables, enabling full duplex communications. 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. The actual baud rate produced differs from that requested by a small error owing to rounding of an internal divisor. The UART Wildcard supports any baud rate produced by the above formula. Each of the two channels on the UART Wildcard can be configured for RS232, RS422, or RS485. Data translation between different machines can be performed with ease, and applications that communicate via the one serial port can be debugged using the other serial channel. The UART Wildcard implements these optional RS232 modem handshaking signals on channel 1. The handshaking signals can be disabled and/or ignored by applications that do not need them. 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.

In this case, cable connections must be made to Serial 1 at pins 5 and 6 of the 10-pin Serial Header or pins 7 and 8 on the 24-pin Field Header. Any required SPI output signals must be configured as outputs, either by calling InitSPI() or by setting the appropriate bits in the Port D data direction register DDRD. The M bit, with mask 0x10, determines whether eight or nine bits total are transmitted with each byte, regardless of whether or not the most-significant bit is a parity bit. 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. Even parity means that the bits sum to an even number, and odd parity means that the bits sum to an odd number. High (mark) parity means that the parity bit is always logic 1 at the UART, and low (space) parity means that the parity bit is always logic 0 at the UART.

At the UART, a logic high (5 volts) is called a mark, and a logic low (0 volts) is called a space. 12 volts and outputs logic level (0 or 5 volt) signals to the UART circuitry. If the /SS pin of the master is an input and if a low input level is detected, the processor sets the MODF bit in the SPI status register a "mode fault" condition. If you are using the QScreen as a slave device and require the /SS signal for your external SPI hardware, configure one of the Port A pins on the Field Header as an input pin. The SPE bit turns on the SPI system. A logic-low start bit marks the start of a character, followed by 5 to 8 data bits per character. 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.

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