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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. A (green, negative) is higher than B (red, positive). RS232 has numerous handshaking lines (primarily used with modems), and also specifies a communications protocol. 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. Once the project is open, click Build→ Build, and after the compilation is done, enter the Mosaic Terminal by clicking Tools→ Mosaic Terminal and use the Send File menu item to send GETSTART.DLF to the PDQ Board.

If your computer does not have an RS232 serial port, low cost USB-to-RS232 serial cables are available; contact Mosaic Industries for details. 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. It is a half duplex protocol, meaning that only one party at a time may transmit data. 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. Although data byte transfers are easily executed once the network has been wired and configured properly, rs485 cable a carefully executed software protocol may be required to ensure data integrity. As the master transmits a byte to an active slave (that is, a slave with its /SS input active low), the master receives a byte from the slave. The diagram below shows potentials of the A (blue) and B (red) pins of an RS-485 line during transmission of one byte (0xD3, least significant bit first) of data using an asynchronous start-stop method. 7.50 extra for an FTDI module - the one I use has a micro USB connector. Offboard FTDI Module with USB-B Micro, add $8.

Kits less the MIKROE-483 USB module $27. This 6 ft long cable comes with a USB connector on one end and the red and yellow tipped plugs and does not require a driver if used with Windows 7 or later versions. RS422. RS422 is also specified for multi-drop (party-line) applications where only one driver is connected to, and transmits on, a "bus" of up to 10 receivers. A 2-wire synchronous IIC (Inter-IC) bus provides multi-drop signaling at rates up to 100 Kbaud. Thus, as a rough approximation, operating at 4800 baud full duplex requires about 40 to 50% of the 6811's CPU time (that is, an average of approximately 40 to 50 µs service time every 100 µs). The Serial2 channel is always configured for RS232 communications, and can sustain baud rates up to 4800 baud. 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. The Electronics Industry Association (EIA) has produced standards for RS485, RS422, RS232, and RS423 that deal with data communications. Standards have been developed to insure compatibility between units provided by different manufacturers, and to allow for reasonable success in transferring data over specified distances and/or data rates. Any comments and/or corrections would be appreciated. Any number of characters can be sent, and the transmitter will automatically re-trigger with each new character (or in many cases a "bit-oriented" timing scheme is used in conjunction with network biasing for fully automatic operation, including any Baud rate and/or any communications specification, eg. Each UART (sometimes referred to as a "USART") controls the serial-to-parallel and parallel-to-serial conversion and performs all of the timing functions necessary for one asynchronous serial communications link. 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.