The GROUND line serves as a common voltage reference for the master and slave. The RS485 employs a set of 120 Ohm termination resistors located at each end of the line. A jumper, J3, configures the primary serial port for either RS232 or RS485 operation. RS485 is another protocol supported by the primary serial port on the QScreen Controller. Unlike the standard RS232 protocol, RS485 allows many communicating parties to share the same 3-wire communications cable. The standard C serial I/O routines such as printf(), scanf(), putchar(), and getchar() give you high level access to the serial ports. The serial output at the UART idles at the logic high (mark) level. Because all of the serial I/O routines on the QScreen Controller are revectorable, it is very easy to change the serial port in use without modifying any high level code. 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.

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 secondary serial port is connected similarly except that the onboard connection of RTS to CTS, and DSR to DTR are permanent. 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. It is supported by virtually all personal computers, and is the default protocol for both of the QScreen Controller’s serial ports. 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 primary and secondary serial communications ports are accessible through the QScreen's 10 pin, dual row Serial Header (H5) which is typically not installed, the 24 pin, dual row Field Header (H3), and through the individual DB-9 Serial 1 and Serial 2 connectors. Slave devices use the master in/slave out pin, MISO, for transmitting, and the master out/slave in pin, MOSI, for receiving data.

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 only difference between the master and slave devices is that the master initiates the transmission. In order to avoid errors when many devices are connected, cables of the same colour should be used for all the connections to the terminals A and cables of the same colour should be used for all the connections to the terminals B of the various devices (e.g. white for A and blue for B). The RS485 connections are not brought out to the Serial 1 Connector. It is interesting to measure the humidity of the ground, but we have also thought about other things; and then we have TXtemp to measure the temperature (one of the uses of TXtemp is in association with SuperClock), and for practically any type of measurement we have TXdata (RS232, RS485 ModBus) - which can be connected to thousands of existing third-party professional RS232 or ModBus meters (and there is something for all needs: temperature and humidity meters, anemometers, voltage meters, energy, light meters, water meters, presence sensors, brightness sensors, soil humidity sensors, etc. ; practically all professional meters are available in ModBus or RS232 standard; and not just meters).

5 Volt supply by a dual RS232 driver chip that has a built-in charge pump voltage multiplier. The RS485 protocol uses differential data signals for improved noise immunity; thus RS485 can communicate over greater distances than RS232. Custom FTDI USB - RS485 Cable with Molex connector, fits all GasLab EC and MX Smart Sensor Boards and Development Kits. 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. 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). By polling the Port A pin or by setting up an interrupt service routine, you can configure the QScreen to ignore the SCK input when /SS is high and keep MISO in a high-impedance state so that it does not interfere with the SPI bus. In the middle of a character, each interrupt service routine takes about 35 µs.