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If an interrupt service routine takes longer than 200 µs, then an entire serial bit will be missed, causing a communications error. From the QScreen Controller’s point of view, these three signals (TxD, RxD, and ground) are the only connections required to perform serial communications. To provide a convenient means of attaching two grounds to the serial cable, there are several pins (labeled GND) on the communications connector that are connected to the controller’s ground plane. In this case, cable connections may be made to Serial 1 on either the 10-pin Serial Communications Header or the Serial 1 Connector. In this case, cable connections must be made to Serial 1 at pins 5 and 6 of the 10-pin Serial Header or pins 7 and 8 on the 24-pin Field Header. The pinout of the QScreen’s Serial Header (H5), QScreen’s Field Header (H3), and the Serial Connectors are shown in the following tables. There are surface mount resistor pads on the QScreen that will allow you to bring out the secondary serial port to the Field Header on pins 5-6 or 7-8 as shown with the parentheses in Table 11-3. Pads are also available to bring out the RS485 signals to the DB9 Serial 1 Connector.

On the other hand, the secondary serial port (Serial2) is implemented using hardware pins PA3 (input) and PA4 (output), and is controlled by the associated interrupts IC4/OC5 and OC4, respectively. 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. 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! If your application requires RS485, use the primary serial port (serial1) for RS485 communications, and use the secondary serial port (Serial 2) to program and debug your application code using the RS232 protocol. Using the primary serial port is easy. In fact, you have been using it all along as you worked through the examples in this document. Much like RS232, RS485 doesn’t have an actual standard connector. In fact, there is no such thing as an RS485 cable since RS485 is a protocol for serial communication (like between inverter and Zero export device) and Ethernet is a protocol for carrying internet data (between router and inverter) whereas CAT6 is a cable standard.

This is typically handled by the protocol being used on top of RS-485, such as Modbus or Profibus.For example, in a Modbus network, each message sent by the master starts with the address of the device it’s intended for. Just like it’s older brother RS232, RS485 is a form of serial communication. 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. Pre-coded device drivers configure the SPI for a standard data format, what is rs485 cable and it is easy to customize a data format and baud rate for your application. 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. 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. If you are running Serial2 at 4800 baud, the rest of your application must be able to function properly using the remaining portion of the CPU time.

1200 is the baud rate that you choose; you can specify any standard baud rate up to 4800 baud. Now select the "Comm" item in the "Settings" menu of the Terminal program, and click on 1200 baud (or whatever baud rate you selected in the command above). 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. The maximum Serial2 communications rate is 4800 baud. The rate of data transmission is expressed in bits per second, or baud. You can operate the port at any baud rate up to 4800 baud; just specify the rate you want before the BAUD2 command. The Serial 2 port is dedicated to RS232 communications at up to 4800 baud. Given the availability of ready-made communications cables, it is not necessary to study or understand the following descriptions of cable connections. You aren’t limited to fifty feet of cable like with RS232.