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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. 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. Assembled cards are $21.50 and ship flat rate boxes. You can operate the port at any baud rate up to 4800 baud; just specify the rate you want before the BAUD2 command. 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. After configuring the SPI system to communicate on a properly connected network of devices, sending and receiving data is as simple as writing and reading a register. After a data transfer is initiated by writing to the SPDR data register, the processor may poll the SPSR status register until the SPIF flag is set.

Given a properly wired network and a properly configured SPCR control register, a master device may transmit a message by simply storing the byte to the SPDR data register. The device that initiates a data transfer is the master, and all other devices on the network are slaves. 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". It provides a convenient means of connecting the QScreen Controller to a variety of peripheral devices, including analog to digital and digital to analog converters, real time clocks, and other computers which use high speed communication. When the QScreen controls the network, it is referred to as a "master"; otherwise, it is a "slave". Configured as a master device, the QScreen transmits bytes via the "master out/slave in" pin, MOSI. We have built sophisticated instruments using the QScreen Controller that operate very reliably using multiple interrupts in addition to the software UART.

In addition to its basic function as a camera test monitor, the CCTVmate has a number of built in test meters and is a professional troubleshooting kit for any CCTV engineer. The diagram below shows a basic RS485 cable for use with the RJ11 socket on Unitronics PLCs. The diagram below shows potentials of the A (blue) and B (red) pins of an RS-485 line during transmission of one byte (0xD3, rs485 cable least significant bit first) of data using an asynchronous start-stop method. The SCK pin clocks the serial A/D’s CLK input which causes the A/D’s conversion result to be transferred to the master via the MISO line. Remember that the /SS is active low so to select a device you need to set the pin low; otherwise the pin should idle high. The SCK (serial clock) pin is a configurable synchronous data clock output. Before running the program, let’s switch to the secondary serial port. Now select the "Comm" item in the "Settings" menu of the Terminal program, and click on 1200 baud (or whatever baud rate you selected in the command above). 1200 is the baud rate that you choose; you can specify any standard baud rate up to 4800 baud.

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. 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). If you are running Serial2 at 4800 baud, the rest of your application must be able to function properly using the remaining portion of the CPU time. However the other standards that have been introduced more recently provide higher levels of performance that are very useful in may applications. RS-485, also known as TIA-485(-A) or EIA-485, is a standard, originally introduced in 1983, defining the electrical characteristics of drivers and receivers for use in serial communications systems. 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 GROUND line serves as a common voltage reference for the master and slave. A ground connection is also necessary to ensure that the communicating devices have a common voltage reference.