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RS485 extends the common mode range for both drivers and receivers in the "tri-state" mode and with power off. The RS232 signals are represented by voltage levels with respect to a system common (power / logic ground). The "idle" state (MARK) has the signal level negative with respect to common, and the "active" state (SPACE) has the signal level positive with respect to common. The serial output at the UART idles at the logic high (mark) level. Now you can stock just one adapter to fill all your legacy "RS" serial interfacing plus provide a low level TTL control interface. This page describes the serial ports and how to use them for instrument control and automation applications. Only one active master may control the network at a time; however, the device that assumes the role of master may change according to an appropriate protocol. 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. In this case, cable connections may be made to Serial 1 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 Serial1 DB-9 Connector.

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. After a data transfer is initiated by writing to the SPDR data register, the processor may poll the SPSR status register until the SPIF flag is set. Finally, for master devices, the SPR1 and SPR0 bits determine the baud rate at which data is exchanged. 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. They translate the bit-by-bit data on the serial cable into bytes of data that can be interpreted by the operating system or by your application program. Electronic data communications between elements will generally fall into two broad categories: single-ended and differential. When communicating at high data rates, or over long distances in real world environments, single-ended methods are often inadequate. RS232 (single-ended) was introduced in 1962, and despite rumors for its early demise, has remained widely used through the industry. RS423 is another single ended specification with enhanced operation over RS232; however, it has not been widely used in the industry.

A true multi-point network consists of multiple drivers and receivers connected on a single bus, rs485 cable where any node can transmit or receive data. 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). The Serial ports are implemented by the dual on-chip hardware UARTs (Universal Asynchronous Receiver/Transmitters) on the Freescale 9S12 (HCS12) microcontroller. The PDQ Single Board Computer (SBC) has two asynchronous serial communications ports named Serial1 and Serial2. Typically one device (node) is addressed by the host computer and a response is received from that device. If your computer does not have an RS232 serial port, low cost USB-to-RS232 serial cables are available; contact Mosaic Industries for details. The QScreen allows the details of the synchronous communications protocol to be customized for compatibility with a variety of peripherals. The QScreen Controller controls the RS485 transceiver with bit 5 of Port D of the processor. There are a variety of ways the MOSI, MISO, SCK and /SS pins on your QScreen Controller can be connected. By setting this output LOW, the slave’s input /SS is pulled LOW.

It is important to note that when the CPHA bit is 0, the /SS line must be de-asserted and re-asserted between each successive data byte exchange (68HC11 Reference Manual, Section 8.3.2). If the CPHA bit is 1, the /SS line may be tied low between successive transfers. In its simplest form, a pair of converters from RS232 to RS422 (and back again) can be used to form an "RS232 extension cord." Data rates of up to 100K bits / second and distances up to 4000 Ft. 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. Two asynchronous communications ports named Serial1 and Serial2 can each be configured for RS232 or RS485 protocols. The primary and secondary serial communications ports are accessible through the PDQ Board's 10 pin, dual row Communications Header (H2) and through the Docking Panel's 10 pin, right-angle, dual row Communications Header (H1) and individual DB-9 Serial 1 and Serial 2 connectors. Also, RS485 drivers are able to withstand "data collisions" (bus contention) problems and bus fault conditions.