RS485 - what is RS485, EIA-485

Without termination resistors, reflections of fast driver edges can cause multiple data edges that can cause data corruption. This allowed for electrical noise to cause interference. To reduce the effect of interference from external electrical signals the RS485 interface uses a differential signal over a "twisted pair" cable. RS485 can handle up to 32 connected devices. With careful design, many peripherals can communicate via the SPI, and powerful multi-processor systems can be linked using this high speed 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. The QScreen allows the details of the synchronous communications protocol to be customized for compatibility with a variety of peripherals.

3. Could you do customized functions according to requirements ? RESOURCE and the access control functions GET() and RELEASE(). However, note that the functions that write to EEPROM disable interrupts for 20 msec. Since RS-485 is a multi-point specification, however, this is not necessary or desirable in many cases. Among various topological structures, the Daisy Chain topology is considered the optimal choice for RS-485 due to its minimal impact on signal integrity. This signal synchronizes the exchange of bytes between the QScreen and its peripherals. If you are using the QScreen as a master device, each external SPI device will require a separate select line (/SS). The BufferToSPI() function implements fast data transfer from a specified buffer in the controller’s memory to an SPI device. Consult the data sheets for any peripheral devices that you are interfacing to the SPI and, if a different configuration is needed, follow the instructions below to set up the appropriate SPI data transfer protocol. Only one active master may control the network at a time; however, the device that assumes the role of master may change according to an appropriate protocol. What is the RS485 protocol used on Monicon's controllers?

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. The RS485 network must be designed as one line with multiple drops, not as a star or ring topology. Ring topology arranges network nodes in a circular configuration where each node connects to two others, forming a closed loop. The two lowest order bits in the SPCR control register, named SPR1 and SPR0, determine the data exchange frequency expressed in bits per second; this frequency is also known as the baud rate. The two-wire half-duplex system enables bidirectional data transmission between two devices, but not simultaneously. Finally, for master devices, the SPR1 and SPR0 bits determine the baud rate at which data is exchanged. There are three flag bits implemented in the SPSR (SPI status register). Hardware is interfaced to the SPI via three PORTD pins named SCK, MOSI, and MISO brought out to pins 7, 8, and 10 on the Wildcard Port Header (see Appendix B). It receives bytes sent by a slave device via the "master in/slave out" pin, MISO. If not, check out our other blog post What is RS232. You aren’t limited to fifty feet of cable like with RS232.

RS232 could only handle one. Note that the data is changed by the transmitting device one half clock cycle before it is valid. The status of a device as master or slave determines how the various pins must be configured. Care must be taken when using A/B naming. Care must be taken that an SC connection, especially over long cable runs, does not result in an attempt to connect disparate grounds together - it is wise to add some current limiting to the SC connection. 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. These steps greatly reduce the chance that the communicating devices might be damaged by contention on the SPI bus.

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