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. However, note that the functions that write to EEPROM disable interrupts for 20 msec. In some cases, however, a sophisticated network may have device groups on a network that use different clock configurations. For example, the user may focus on MODBUS or PPP protocol frame encapsulated in serial protocol frame. The Serial PPP View visualizer can be used to decode Point-to-Point Protocol packets, as well as a number of packets that can be encapsulated in PPP packets.PPP View supports the following protocols: Novell IPX, OSI Network Layer Control Protocol, DECnet Phase IV Control Protocol, Appletalk Control Protocol, Link Control Protocol, Password Authentication Protocol, Link Quality Report, Challenge Handshake Authentication Protocol, Multilink Protocol, Compression Control Protocol, Extensible Authentication Protocol, NETBIOS Protocol. The main difference between RS422 and RS485 is that RS422 supports point-to-point communication, while RS485 allows for multidrop networks where multiple devices can share the same bus. 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".

To ensure that no two devices drive the network at the same time, it is necessary that each slave device be able to disable it’s own RS-485 data transmitter. It’s important to note that all devices on the network need to use the same logic for interpreting voltage differences as bits (i.e., whether A being at a higher voltage than B represents a ‘1’ or a ‘0’).In a network with multiple devices, each device needs to have a unique address so that it knows when to listen and when to ignore the communications on the line. This helps to minimize the impact of electromagnetic interference, causing the voltage between the two wires to deviate. In RS422 and RS485, the idle state is a "mark" (positive differential voltage). 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 characteristic feature of the Daisy Chain topology is the sequential connection of each device along a single line, not forming a loop structure. RS485 industry Standard use requirements, so when the distance between each device line is relatively long, it is easy to cause problems such as signal reflection and anti-interference ability, which leads to a decrease in the reliability of the control signal.

You can use the QScreen’s RS485 link to create such a multi-drop serial network. Connecting computers together in multi-drop networks is common in factories and laboratories. Connecting the RS485 leads to a DVR is fairly simple. The configuration and specifications of RS485 make it faster and extend the range of data transmission. 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. 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. Although the maximum standard baud rate of the primary serial port is 19200 baud, nonstandard baud rates of over 80 Kbaud can be attained by the 68HC11's on-chip UART and the onboard RS232 driver. 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. RS232 communication works by transmitting data one bit at a time (serially) over a communication channel. The Serial2 channel is always configured for RS232 communications, and can sustain baud rates up to 4800 baud. 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.

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. 70 as a best case and 100 as a worst case (obviously reading descriptions will take more) multiply by the number of objects and properties you are going to poll on a regular basis. 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. This section describes the QED-Forth routines that control the RS485 transceiver, and presents some ideas that may prove useful in designing a multi-drop data exchange protocol. 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 may be wondering, what can I do with serial communications, especially RS485. 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.

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