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The communications port also contains pins for the bi-directional encoder bus. It does not specify or recommend any communications protocol; Other standards define the protocols for communication over an RS-485 link. 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 secondary serial port is connected similarly except that the onboard connection of RTS to CTS, and DSR to DTR are permanent. The maximum sustainable baud rate on the secondary serial port is 4800 baud. The maximum Serial2 communications rate is 4800 baud. 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. 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. In fact, the program works the same as it did before, but now it is using the secondary serial port instead of the primary port -- and you didn’t even have to recompile the code!

In fact, you have been using it all along as you worked through the examples in this document. Done that establishes the RS485 receive mode is coded such that it waits until all queued (pending) characters have been transmitted before the driver chip is taken out of transmit mode. Custom FTDI USB - RS485 Cable with Molex connector, fits all GasLab EC and MX Smart Sensor Boards and Development Kits. The value of each termination resistor should be equal to the cable characteristic impedance (typically, 120 ohms for twisted pairs). By connecting pairs of these handshaking signals together, the terminal or PC can be made to think that the QScreen Controller is always ready to send and receive data. If you have not yet compiled the GETSTART program and you want to do the exercises here, open GETSTART.C in your TextPad editor, click on the Make Tool, and after the compilation is done, enter Mosaic Terminal by clicking on the terminal icon and use the "Send File" menu item to send GETSTART.DLF to the QScreen Controller. Typing a carriage return at the terminal should now produce the familiar "ok" response via the Serial2 port. The Serial2 channel is always configured for RS232 communications, and can sustain baud rates up to 4800 baud.

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. In this case, cable connections may be made to Serial 1 on either the 10-pin Serial Communications Header or the Serial 1 Connector. The QScreen Controller’s transmit data signal TxD1 (pin 2 on the 9-pin serial connector) is connected to the terminal’s receive data signal RxD (pin 2 on its 9-pin connector). Likewise, the terminal’s transmit signal TxD is connected to the QScreen Controller’s receive signal RxD1. If you do this now, remember to move the QScreen Controller’s serial connector back to Serial Port 1, and to change the terminal’s baud rate back to 19200 baud using the "Comm" item under the terminal’s "Settings" menu. Move the serial cable from the "Serial Port 1" connector to the "Serial Port 2" connector on the QScreen. The cable is shielded to further protect against interference and noise.

For fixed laying Maximum shielding capacity in accordance with EMC requirements, Cable for Modbus bus systems, SUCOnet P, Modulink P, VariNet-P, Dry and damp environments, Suitable for various bus systems based on RS485 / RS422. Henceforth, we shall consider Slave devices to be measuring instruments with serial communication, even if the cabling is similar for all Modbus devices. 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. The default serial routines used by the onboard kernel assume that full duplex communications are available, so you cannot use the RS485 protocol to program the controller. Although many applications use RS-485 signal levels, the speed, format, and protocol of the data transmission are not specified by RS-485. Rather, it relies on software handshaking via transmission of XON/XOFF characters to coordinate data transfer and ensure that information is not lost when one of the communicating parties is busy.