Parity options include even, odd, high, low, or no parity. When this bit is high, the transceiver is in transmit mode. Like RS232, the data bits are transmitted in reverse order, with the least significant bit transmitted first. By connecting pairs of these handshaking signals together, the terminal or PC can be made to think that the QScreen Controller is always ready to send and receive data. Most modems communicate using RS232 and a set of hardware handshaking signals used to regulate data flow. The Serial 1 port can be configured for either RS232 or RS485 communications at up to 19200 baud. The words port and channel are used interchangeably to refer to a serial communications link. From the PDQ Board’s point of view, these three signals (/TxD, /RxD, and ground) are the only connections required to perform serial communications. In this case, cable connections may be made to Serial 2 on either the 10-pin PDQ Board Serial Communications Header, or the Docking Panel’s 10-pin right-angle Serial Header, or the Docking Panel’s Serial2 DB-9 Connector. If this limitation is not a problem, you can reverse the roles of the Serial1 and Serial2 ports, because they have identical communications capabilities. Using the primary Serial1 port is easy.

RS485 is another protocol supported by the primary serial port on the QScreen Controller. There are surface mount resistor pads on the QScreen that will allow you to bring out the secondary serial port to the Field Header on pins 5-6 or 7-8 as shown with the parentheses in Table 11-3. Pads are also available to bring out the RS485 signals to the DB9 Serial 1 Connector. If your application requires RS485, you can use the secondary serial port (serial2) to program and debug your application code using the RS232 protocol, and use the primary serial port (Serial1) for RS485 communications. 5 Volt supply by a dual RS232 driver chip that has a built-in charge pump voltage multiplier. A jumper labeled "2 485En" (J7) enables RS485 operation on the Serial2 port if the jumper cap is installed, and configures Serial2 for RS232 operation if the jumper cap is not installed. The adapter includes a 3 foot 'type A' cable connection that plugs into any PC or MAC USB port. The secondary serial port is connected similarly except that the onboard connection of /RTS to /CTS, and /DSR to /DTR are permanent.

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! The on-board sync separator produces an interrupt upon detection of a vertical sync pulse which is readable on the serial port. Parity checking is not often used, because it is not a robust method of error detection. 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. One to two logic-high stop bits mark the end of a character. Compatible with all formats, data/baud rates, and bits per character. 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.

Rather, the UART deduces the correct time to sample the incoming signal based on the start and stop bits in the signal itself. In this example, the QScreen Controller selects the serial A/D by outputting a LOW signal on /SS. There are a variety of ways the MOSI, MISO, SCK and /SS pins on your QScreen Controller can be connected. There are surface-mount resistor pads on the Docking Panel to bring out the RS485 signals to the DB9 Serial 1 Connector. Because we chose the default baud rate (which the terminal is presumably already set for), you can simply move the serial cable from the Serial Port 1 connector to the Serial Port 2 connector on the Docking Panel to complete the change to the new port. A jumper labeled "1 485En" (J4) enables RS485 operation on the Serial1 port if the jumper cap is installed, and configures Serial1 for RS232 operation if the jumper cap is not installed. 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).

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