This robustness is the main reason why RS-485 is well suited for long-distance networking in noisy environment. In order to avoid signal reflections, a 120 Ohm termination resistance must be fitted on each end of the main cable. The end devices are responsible for terminating the cable so that there are no reflections from the cable ends. Two devices are at the ends of the cable, while others are connected somewhere in between. The advantage of using Serial1 for RS485 is that the Serial1 RS485 signals are also available on the Docking Panel, while the Serial2 RS485 signals are available only on the PDQ Board’s Serial Communications Header. Serial1 port the default startup serial link. Using the primary serial port is easy. In fact, the program works the same as it did before, but now it is using the secondary serial port instead of the primary port - and you didn’t even have to recompile the code! In either of these cases, a source of noise that caused one bit to be received incorrectly would invalidate the received byte, since the total number of '1' bits would be odd rather than even.

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. It receives bytes sent by a slave device via the "master in/slave out" pin, MISO. As the master transmits its data, 8 bits of data are simultaneously received. The SCK pin clocks the serial A/D’s CLK input which causes the A/D’s conversion result to be transferred to the master via the MISO line. Also, several non-serial interrupts can stack up; if they have higher priority than the serial interrupts, they will be serviced before the Serial2 interrupt routine, and again a serial input or output bit may be lost. In the middle of a character, each interrupt service routine takes about 35 µs. In most cases the times are less than 25 µs which does not pose a problem. Although data byte transfers are easily executed once the network has been wired and configured properly, a carefully executed software protocol may be required to ensure data integrity.

Rather, it relies on software handshaking via transmission of XON/XOFF characters (ascii 0x11 and 0x13, respectively) to coordinate data transfer and ensure that information is not lost when one of the communicating parties is busy. The RS232 protocol provides for four handshaking signals called ready to send (RTS), clear to send (CTS), data set ready (DSR), and data terminal ready (DTR) to coordinate the transfer of information. Once the data has been exchanged, a flag bit in the SPSR status register is set to indicate that the transfer is complete. 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. This ability to exchange messages means that the SPI is capable of full duplex communication. The RS422 driver and receiver use separate differential conductor pairs on the serial cables, enabling full duplex communications. The RS232 protocol specifies the use of two separate grounds, a signal ground and a protective (or chassis) ground. Although the RS232 protocol specifies functions for as many as 25 pins, each communications channel requires only three for simple serial interfaces: /TxD1 (transmit data), /RxD1 (receive data), and DGND (digital ground).

The Serial2 channel is always configured for RS232 communications, and can sustain baud rates up to 4800 baud. Each of the two channels on the UART Wildcard can be configured for RS232, RS422, or RS485. Depends what you mean by a serial cable - Fibre, Ethernet, RS232, RS485, RS422, USB, HDMI, SMPTE292 are all examples of serial communications media. No termination - If the PDQ Board is not an end device, you should not terminate that cable. Most computers conform to IBM PC AT-compatible RS232 interfaces which use 9-pin D-Type connectors, consequently the PDQ Board brings out its serial ports to two female 9-pin D-Type connectors on the Docking Panel. In a computer system, SCSI-2 and SCSI-3 may use this specification to implement the physical layer for data transmission between a controller and a disk drive. In this case, cable connections may be made to Serial 1 at pins 5 and 6 of the PDQ Board’s 10-pin Serial Header , or pins 5 and 6 of the Docking Panel’s 10-pin right-angle Serial Header. For this reason, frame-level cyclic redundancy checks are much more widely used for validating data from serial links, network connections and storage media.

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