Don't Waste Time! 5 Information To begin What Is Rs485 Cable > 자유게시판

The standard does not discuss cable shielding but makes some recommendations on preferred methods of interconnecting the signal reference common and equipment case grounds. RS-485 is also used in building automation as the simple bus wiring and long cable length is ideal for joining remote devices. We also highly recommend using paired twisted cable for connecting most converters. If multiple analog input meters or transmitters are digitally addressed in command mode on an RS485 line using the Modbus RTU protocol or the Custom ASCII protocol, the rate becomes about 2 readings per second. The reason is that the serial RS232 protocol is, even its old, a very reliable, stable and solid communication protocol, which can easily be implemented with existing industrial equipment. 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. A ground connection is also necessary to ensure that the communicating devices have a common voltage reference. We have built sophisticated instruments using the QScreen Controller that operate very reliably using multiple interrupts in addition to the software UART. When the QScreen controls the network, it is referred to as a "master"; otherwise, it is a "slave".

The typical serial converter does this in both directions which is also referred to as a bi-directional converter. So in order to transfer data from a RS232 serial interface engineers need a converter to be able to connect to a RS485 or RS422 interface. So what is a RS232 to RS485 converter? Using custom protocol feature you can parse any SCADA industrial protocol which is transmitted over RS485, RS422 or RS232 serial connection, dial up/radio modem or TCP/IP, UDP/IP network. Because the software UART is interrupt based, competing interrupts that prevent timely servicing of the Serial2 interrupts can cause communications errors on the secondary serial channel. The maximum Serial2 communications rate is 4800 baud. The rate of data transmission is expressed in bits per second, or baud. The SPI can transfer data much more rapidly than an asynchronous serial link - its maximum rate is 2 Megabits/second. For the QScreen, /SS is not used for SPI communication because it is used to control the direction of the RS485 transceiver; you can use any digital I/O line as a /SS signal. A mode fault occurs when the SPI senses that a multimaster conflict (MC68HC11F1 Technical Data Manual, p.8-7) exists on the network as explained above in connection with the /SS input.

A write collision occurs when a byte is written to the SPI data register, SPDR, while data is being exchanged. The SPIE bit in the SPCR (SPI control register) enables SPI interrupt handling. 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. In this section we will consider the most general and simple configurations. 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. Instead of exposing data as individual packets, as most other visualizers do, it groups data according to the underlying text protocol. The data exchange format may be a line of ascii text. It is important to note that when the CPHA bit is 0, the /SS line must be de-asserted and re-asserted between each successive data byte exchange (68HC11 Reference Manual, Section 8.3.2). If the CPHA bit is 1, the /SS line may be tied low between successive transfers. It may be that only the byte sent from the master to the slave is meaningful; nevertheless, each device simultaneously transmits and receives one byte.

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. To use a QScreen as a slave in a multi-drop network, simply define a word, (named Silence(void), for example) that when executed calls RS485Receive() to wait for any pending character transmission to complete, then disable the transmitter, and then execute a routine such as Key() to listen to the communications on the serial bus. 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. 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. If an interrupt service routine takes longer than 200 µs, then an entire serial bit will be missed, causing a communications error. With this unique feature of our product, you can snoop data even on remote serial ports. Routines that temporarily disable interrupts for significant periods of time can also interfere with the Serial2 port.

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