Rather, it relies on software handshaking via transmission of XON/XOFF characters (ascii 0x11 and 0x13, rs485 cable respectively) to coordinate data transfer and ensure that information is not lost when one of the communicating parties is busy. Alarm systems: Rs485 cables in alarm systems help transfer alarm signals and information from alarm devices to the control center. The advantage of the RS485 protocol is that this it can transfer data over longer distances, up to 4000 feet, and have up to 32 nodes per single network, and it can be used for multi-point networks where multiple serial devices are setup in a bus network. If your application requires RS485, you can use the secondary serial port (serial2) to program and debug your application code using the RS232 protocol, and use the primary serial port (Serial1) for RS485 communications. Using the primary Serial1 port is easy. We assume that you are now communicating with the PDQ Board via the default Serial1 port at the standard 115200 baud rate.
If it doesn’t, confirm that the terminal’s baud rate is correct by selecting the Comm item in the Settings menu of the Mosaic Terminal program, and click on 115200 baud. Because we chose the default baud rate (which the terminal is presumably already set for), you can simply move the serial cable from the Serial Port 1 connector to the Serial Port 2 connector on the Docking Panel to complete the change to the new port. The default serial routines used to download programs to the operating system assume that full duplex communications are available, so you cannot use the RS485 protocol to program the controller. The advantage of using Serial1 for RS485 is that the Serial1 RS485 signals are also available on the Docking Panel, while the Serial2 RS485 signals are available only on the PDQ Board’s Serial Communications Header. In this case, cable connections may be made to Serial 1 at pins 5 and 6 of the PDQ Board’s 10-pin Serial Header , or pins 5 and 6 of the Docking Panel’s 10-pin right-angle Serial Header.
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). In this case, cable connections may be made to Serial 2 at pins 4 and 10 of the PDQ Board’s 10-pin Serial Header, or pins 5 and 6 of the Docking Panel’s 10-pin right-angle Serial Header. The serial data stream at the UART is conditioned by serial driver chips that transmit and receive the data. That places a 120 Ω resistor across the RS485 differential line at the driver chip. By default, the RS485 connections are not brought out to the Docking Panel’s DB-9 Serial1 Connector, although custom placement of zero-ohm surface-mount resistors on the Docking Panel can route the RS485 signals to the DB-9. By default, the RS485 connections are not brought out to the Docking Panel’s DB-9 Serial1 Connector.
Two devices are at the ends of the cable, while others are connected somewhere in between. The end devices are responsible for terminating the cable so that there are no reflections from the cable ends. Electrical signaling is balanced, and multipoint systems are supported. 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. That's actually using a 485 transceiver in the box. In that case, when using very long cables you can improve noise immunity and assure a valid idle level when the transceiver is not active by installing bias resistors. In the simplest scheme, all RS485 transceivers come up in receive mode when the interface is initialized, and each transceiver node has a unique address known to it and the master. The GROUND line serves as a common voltage reference for the master and slave. Connecting computers together in multi-drop networks is common in factories and laboratories. RS485 multi-drop networks are daisy-chained networks with a single cable connecting multiple devices.
