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Thus in Table 9 6 , RTS1 is connected to CTS1, and DSR1 is connected to DTR1 and DCD1 onboard the QScreen Controller using zero ohm shorting resistors. The RS485 protocol uses differential data signals for improved noise immunity; thus RS485 can communicate over greater distances than RS232. A jumper, J3, configures the primary serial port for either RS232 or RS485 operation. 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. Since both channels can operate simultaneously and independently, debugging can be performed while the application program is communicating via its primary channel. After having defined the type of instrument best suited to the needs, if the measurement is performed by indirect insertion, the accessories of the measuring system, such as current and voltage transformers must be chosen with care. This is due to USBs’ advantages of being faster, having lower voltages, and having connectors that are simple to connect and use. The fastest RS-485 transceivers out there right now are running about 50 megabits. Before running the program, let’s switch to the secondary serial port.

The Serial 2 port is dedicated to RS232 communications at up to 4800 baud. The RS232 protocol provides for four handshaking signals called ready to send (RTS), clear to send (CTS), data set ready (DSR), and data terminal ready (DTR) to coordinate the transfer of information. Many terminals and PCs, however, do rely on hardware handshaking to determine when the other party (in this case the QScreen Controller) is ready to accept data. It is a half duplex protocol, meaning that only one party at a time may transmit data. Rather, the UART deduces the correct time to sample the incoming signal based on the start and stop bits in the signal itself. Because of this RS485 and the others can be used in situations with a severe ground level shift of several volts, where at the same time high bit rates are possible because the transition between logical 0 and logical 1 is only a few hundred millivolts. Although data byte transfers are easily executed once the network has been wired and configured properly, a carefully executed software protocol may be required to ensure data integrity. And that with an interface which does not require intelligent network hardware: the implementation on the software side is not much more difficult than with RS232.

RS232 allows both communicating parties to transmit and receive data at the same time; this is referred to as full duplex communications. The default serial routines used by the onboard kernel assume that full duplex communications are available, so you cannot use the RS485 protocol to program the controller. These detailed signal descriptions and cable diagrams are presented to provide complete information for those who have special communications requirements and for those who wish to make their own application-specific communications cables. A serial communications cable is also supplied with QScreen Starter Kits. Its simplest implementation requires only three wires: one to transmit serial data, a second to receive serial data, and a third to provide a common ground reference. 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). If your application requires RS485, use the primary serial port (serial1) for RS485 communications, and use the secondary serial port (Serial 2) to program and debug your application code using the RS232 protocol.

Most computers conform to IBM PC AT-compatible RS232 interfaces which use 9-pin D-Type connectors, consequently the QScreen Controller brings out its serial ports to two female 9-pin D-Type connectors. The RS485 connections are not brought out to the Serial 1 Connector. 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. 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. The QScreen Controller combines an embedded computer based on the 68HC11 microcontroller with a touch panel and LCD (liquid crystal display) graphic user interface (GUI) that is ideal for instrument control and automation. To interface devices that support synchronized serial interfaces, but are not configurable like the 68HC11, determine the device’s requirements for clock phase and polarity and configure the 68HC11’s CPHA and CPOL accordingly. The byte-sized messages are transmitted and received via the MOSI (master out/slave in) and MISO (master in/slave out) pins. Even though the MOSI pin is not connected to anything, the master initiates a transmission using a "dummy" byte.

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