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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. To interface devices that support synchronized serial interfaces, but are not configurable like the QScreen, determine the device’s requirements for clock phase and polarity and configure the QScreen’s CPHA and CPOL accordingly. The device that initiates a data transfer is the master, and all other devices on the network are slaves. It is a half duplex protocol, meaning that only one party at a time may transmit data. Moreover, if Serial2 is running full duplex at 4800 baud, any other interrupt service routine that takes longer than 100 µs is likely to cause a problem. In the middle of a character, each interrupt service routine takes about 35 µs. 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). For example, at 4800 baud (bits per second), each bit lasts about 200 microseconds (µs), and if communications are full duplex (e.g., if the QScreen Controller echoes each incoming character), then there is a serial interrupt every 100 µs or so.

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. We can gain insight into the operation of the RS232 protocol by examining the signal connections used for the primary serial port in Table 9 6. The transmit and receive data signals carry the messages being communicated between the QScreen Controller and the PC or terminal. A jumper, J3, configures the primary serial port for either RS232 or RS485 operation. We recommend that you keep the faster Serial1 port as the default serial link as you work through the exercises in this book. If your application requires use of the secondary serial port as well as other interrupt routines, the key is to keep the interrupt service routines short and fast. The maximum sustainable baud rate on the secondary serial port is 4800 baud. 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! These signals may alternatively be redirected to the digital inputs and outputs used by the second serial port if hardware handshaking is required.

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. 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. Unlike the standard RS232 protocol, RS485 allows many communicating parties to share the same 3-wire communications cable. You can use the QScreen’s RS485 link to create such a multi-drop serial network. In these distributed processing networks, a variety of machines and instruments work locally, but communicate and share data or resources with one another globally using a single serial link. The master and slave can then exchange data. In summary, the code provided for implementing the second serial port is very flexible and can be used to support dual concurrent communications ports. To provide a convenient means of attaching two grounds to the serial cable, there are several pins (labeled GND) on the communications connector that are connected to the controller’s ground plane.

This means that it is protected from contact with all dust particles, and also high-pressured water jets! Please contact Mosaic Industries if you need these custom configurations. In some cases, however, what is rs485 cable a sophisticated network may have device groups on a network that use different clock configurations. Note that the master device outputs the clock synchronization signal SCK to the slave’s SCK which is configured as an input. The two values provide a sufficient margin for a reliable data transmission even under severe signal degradation across the cable and connectors. 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. These characteristics make RS-485 useful in industrial control systems and similar applications. It is commonly implemented to control video surveillance systems and link security control panels to devices such as access control card readers. You can use it to communicate with other devices. Slave devices use the master in/slave out pin, MISO, for transmitting, and the master out/slave in pin, MOSI, for receiving data. The QScreen Controller’s transmit data signal TxD1 (pin 2 on the 9-pin serial connector) is connected to the terminal’s receive data signal RxD (pin 2 on its 9-pin connector).