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In this situation, if the /SS input is pulled low while the 68HC11 is the master, the processor detects a "mode fault" (by setting a bit in the SPI status register) meaning that there is more than one master device on the SPI bus. Regardless of the network, however, there are only four signals used: SCK provides a synchronized clock, MOSI and MISO signals are used for data transmission and reception, and /SS configures the 68HC11 as a master or slave device. It provides a convenient means of connecting the QVGA Controller to a variety of peripheral devices, including analog to digital and digital to analog converters, real time clocks, and other computers which use high speed communication. The QScreen Controller combines an embedded computer based on the 68HC11 microcontroller with a touch panel and LCD (liquid crystal display) graphic user interface (GUI) that is ideal for instrument control and automation. Communications capability is essential for many instrument control applications.

The SPI control register, SPCR, contains 8 bits which must be initialized for proper control of the QVGA Controller’s SPI (M68HC11 Reference Manual, p.8-7). Thus, the master 68HC11 has only one input, MISO, which is the slave QVGA Controller’s only output. The distinction between master and slave is an important one. Not more than one breakout board is required per RS485 line. Question:Does RS485 TO ETH support Modbus transmission? Confirm serial port parameters such as baud rate, data bits, and stop bits: Baud rate should be configured according to the connected RS485 device, commonly used rates are 9600 and 115200; Refer to the device manual or contact RS485 device technical support for configuration, especially if set to Modbus mode, confirm if the function code matches. 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 Serial2 channel is always configured for RS232 communications, and can sustain baud rates up to 4800 baud. The /SS pin can be configured as either an input or an output.

If bit 5 of DDRD is 0, then /SS is an input. If the 68HC11 is initialized as a master (by setting the MSTR bit in the SPCR control register as explained below) then bit 5 of the Port D data direction register (DDRD) determines whether /SS is an input or an output. If bit 5 of DDRD is 1, then /SS is a general purpose output that operates independently of the SPI. Also, in the diagram, the master QVGA Controller’s /SS (slave select) is configured as an output. The next section describes the registers that configure and control the QVGA Controller’s SPI. The next section describes the registers that configure and control the QScreen Controller’s SPI. The QVGA Controller’s on-board 12 bit A/D and 8 bit D/A converters communicate with the processor via the SPI. Serial 2 is implemented by a software UART in the controller’s QED-Forth Kernel that uses two of the processor’s PortA I/O pins to generate a serial communications channel. Hardware is interfaced to the SPI via four PORTD pins named /SS, SCK, MOSI, what is rs485 cable and MISO brought out to pins 11 through 14 on the Digital I/O connector (see Appendix A).

The Serial 1 port is implemented with the 68HC11's on-chip hardware UART (Universal Asynchronous Receiver/Transmitter). 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. This allows the processor that is master to control the input /SS pins of other CPU’s, for example. When the 68HC11 controls the network, it is referred to as a "master"; otherwise, it is a "slave". The secondary serial port is implemented by a software UART that controls two pins on PortA. It controls the serial-to-parallel and parallel-to-serial conversion and performs all of the timing functions necessary for asynchronous serial communications. The Serial 2 port is dedicated to RS232 communications at up to 4800 baud. The Serial 1 port can be configured for either RS232 or RS485 communications at up to 19200 baud. The primary channel’s UART translates the bit-by-bit data on the serial cable into bytes of data that can be interpreted by the QED-Forth Kernel or by your application program. Since both channels can operate simultaneously and independently, debugging can be performed while the application program is communicating via its primary channel. The dual communications channels also provide an easy way to link systems that communicate using different serial protocols.