24 AWG solid wires, which are typically used in Cat 5 cables, can break easily when connected to a screw terminal block, which does not provide any strain relief. Using a repeater, the main cable is divided into different segments, each of which can be up to 700 m in length and connect 32 devices (this number includes the repeaters). In the most common multi-drop RS-485 protocol, one computer is designated as a "master" and the rest of the computers or devices on the serial bus are designated as "slaves". RS-485 is also used in building automation as the simple bus wiring and long cable length is ideal for joining remote devices. If more than one slave tried to drive the transmit line simultaneously, their serial drivers would fight with each other for control of the bus. The remaining "inactive" slaves may actively receive, or listen to, data on the communications line, but only one slave at a time can transmit a message. You can operate the port at any baud rate up to 4800 baud; just specify the rate you want before the BAUD2 command. The primary serial channel can operate at standard speeds up to 19200 baud and can be configured for either RS232 (the default) or RS485 operation.

You can enter the data relating to the MQTT Broker you want to use; the name of the Topic to be used to control it and to read the periodic transmissions of the humidity percentage is shown. TXSoil can transmit the humidity to a Web server or to a VisualVision's display / concentrator such as DisplayRX or ControlHUB or RXTXeasy GSM - with adjustable interval, remaining for the rest of the time in standby with very low consumption. 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. On the other hand, the secondary serial port (Serial2) is implemented using hardware pins PA3 (input) and PA4 (output), and is controlled by the associated interrupts IC4/OC5 and OC4, rs485 cable respectively. Pin 3 of PortA is the Serial2 input, and pin 4 of PortA is the Serial2 output. The QScreen Controller’s kernel software contains a complete set of high level driver routines for the Serial2 port, and these functions are summarized in the Control-C Glossary. 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.

The Silence() routine searches the incoming serial characters for a pre-determined keyword (for example, the ascii "name" of this particular slave). If an interrupt service routine takes longer than 200 µs, then an entire serial bit will be missed, causing a communications error. If a star configuration is unavoidable, special RS-485 repeaters are available which bidirectionally listen for data on each span and then retransmit the data onto all other spans. 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. 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). In most cases the times are less than 25 µs which does not pose a problem. The secondary serial port is connected similarly except that the onboard connection of RTS to CTS, and DSR to DTR are permanent. We recommend that you keep the faster Serial1 port as the default serial link as you work through the exercises in this book.

The primary serial port, Serial1, is supported by the 68HC11's on-chip hardware UART (sometimes called a USART), and does not require interrupts to work properly. 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. 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. Table 9-6 Serial Cable Connections. The standard C serial I/O routines such as printf(), scanf(), putchar(), and getchar() give you high level access to the serial ports. The Control-C Glossary contains a list of functions that temporarily disable interrupts, and the glossary entries give further information regarding how long interrupts are disabled. Rather, it relies on software handshaking via transmission of XON/XOFF characters to coordinate data transfer and ensure that information is not lost when one of the communicating parties is busy. 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.