For those of you interested in the details, here’s how it works: The low-level serial driver routines named Key(), AskKey() and Emit() are revectorable routines that can be redirected to use either of the serial ports. The default serial routines used to download programs to the operating system assume that full duplex communications are available, so you cannot use the rs485 cable protocol to program the controller. Since both channels can operate simultaneously and independently, serial debugging can be performed while the application program is communicating via its primary channel. 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. Rather, the transmitter and receiver must be communicating using a known baud rate, or bit frequency. When the exchange is complete, the slave can again execute the Silence() routine to disable its transmitter and begin listening for its name. The SPIF is set when a data transfer is complete, and is cleared by a read of the SPSR status register, followed by a read or write to the SPDR data register. WCOL is cleared by a read to the SPSR followed by a read or write to the SPDR.

Given a properly wired network and a properly configured SPCR control register, a master device may transmit a message by simply storing the byte to the SPDR data register. These include MUXs, modems, RF up and down converters, and other network components. RS232 is by far the most common serial protocol, and is the default protocol for both of the PDQ Board’s serial ports. In the most common multi-drop RS-485 protocol, one computer is designated as a "master" and the rest of the computers or devices on the serial bus are designated as "slaves". 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. Both the local and remote UARTs must be configured for the same baud rate. The SCK pin’s synchronous clock signal has configurable phase, polarity and baud rate so that it can interface to a variety of synchronous serial devices. In the simplest scheme, all RS485 transceivers come up in receive mode when the interface is initialized, and each transceiver node has a unique address known to it and the master. The master and slave could even exchange ascii QED-Forth commands.

Finally, for master devices, the SPR1 and SPR0 bits determine the baud rate at which data is exchanged. By connecting pairs of these handshaking signals together, the terminal or PC can be made to think that the PDQ Board is always ready to send and receive data. The transmit and receive data signals carry the messages being communicated between the PDQ Board and the PC or terminal. A logic-low start bit marks the start of a character, followed by 5 to 8 data bits per character. InitRS485() configures Port D to ensure that bit 5 is an output. Using the primary serial port is easy. Data translation between different machines can be performed with ease, and applications that communicate via the primary serial port can be debugged using the secondary channel. Many terminals and PCs, however, do rely on hardware handshaking to determine when the other party (in this case the PDQ Board) is ready to accept data. 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.

RS232 uses inverse logic; that is, a positive bit at the HCS12 UART is inverted by the onboard RS232 driver chip and appears as a negative signal on the serial cable. These protocols are summarized on this page, but for more information regarding their data formats and their use for simplex or multi-drop serial lines, consult Understanding Serial Communications (but keep in mind that that page is directed to the use of the UART Wildcard, so it uses different driver functions). A break sequence forces the serial output to a logic low (space) at the UART. If your computer does not have an RS232 serial port, low cost USB-to-RS232 serial cables are available; contact Mosaic Industries for details. All of the RS232 signals start with the / (slash, pronounced not) character to indicate that the signals on the serial cable are logically inverted. The end devices are responsible for terminating the cable so that there are no reflections from the cable ends. No parity means that there is no parity bit. To provide a convenient means of attaching two grounds to the serial cable, there are several pins (labeled DGND) on the communications connector that are connected to the controller’s ground plane.

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