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The connection between two or more elements (drivers and receivers) should be considered a transmission line if the rise and/or fall time is less than half the time for the signal to travel from the transmitter to the receiver. 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 a finished instrument, either or both channels can be used to communicate with other serial devices, or with other computers and/or terminals using RS232 or RS485. Each serial port can be configured for the RS232 or RS485 protocol, and runs at standard baud rates up to 115,200 bits per second. InitRS485() configures Port D to ensure that bit 5 is an output. The BAUD routine described at the start of this subsection configures the baud rate of each of the Serial1 and Serial2 channels.

RS485Init() configures PORTJ to ensure that bits 0 and 1 are outputs, and disables both RS485 transmitters, leaving the Serial1 and Serial2 RS485 channels in receive mode. It is NOT necessary to introduce long delays in a network to avoid "data collisions." Because delays are NOT required, networks can be constructed, that will utilize the data communications bandwidth with up to 100% through put. For this reason, frame-level cyclic redundancy checks are much more widely used for validating data from serial links, network connections and storage media. The Serial ports are implemented by the dual on-chip hardware UARTs (Universal Asynchronous Receiver/Transmitters) on the Freescale 9S12 (HCS12) microcontroller. Both the local and remote UARTs must be configured for the same communications parameters. 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. If two bits are received incorrectly, the error will go unnoticed by parity checking. All of the serial ports are supported by pre-coded C-language software drivers that make it easy to exchange data.

The Serial 1 and Serial2 ports can be configured for either RS-232 or RS-485 communications at standard baud rates up to 115200 bits per second. Two asynchronous communications ports named Serial1 and Serial2 can each be configured for RS232 or RS485 protocols. 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. Once the data has been exchanged, a flag bit in the SPSR status register is set to indicate that the transfer is complete. The PT bit, with mask 0x01, determines whether even parity or odd parity is used if parity bit generation is enabled. 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! With the introduction of "automatic" repeaters and high-impedance drivers / receivers this "limitation" can be extended to hundreds (or even thousands) of nodes on a network.

A true multi-point network consists of multiple drivers and receivers connected on a single bus, where any node can transmit or receive data. Without termination resistors, reflections of fast driver edges can cause multiple data edges that can cause data corruption. In general if you are not connected to a modem the handshaking lines can present a lot of problems if not disabled in software or accounted for in the hardware (loop-back or pulled-up). While these signals provide a data path, they do not provide hardware handshaking that allows the two communicating parties to let each other know when they are ready to send or receive data. Electronic data communications between elements will generally fall into two broad categories: single-ended and differential. If an interrupt service routine takes longer than 200 µs, then an entire serial bit will be missed, causing a communications error. The actual baud rate produced differs from that requested by a small error owing to rounding of an internal divisor. The specification allows for data transmission from one transmitter to one receiver at relatively slow data rates (up to 20K bits/second) and short distances (up to 50Ft. @ the maximum data rate). 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.

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