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. Data translation between different machines can be performed with ease, and applications that communicate via the primary serial port can be debugged using the secondary channel. Since such set is provided only by our product, it is the only one you can use to create inexpensive automatic testing systems for serial equipment and applications. Remember that the /SS is active low so to select a device you need to set the pin low; otherwise the pin should idle high. The /SS (active-low slave select) signal enables data transfers by slave devices when it is active low. For the QScreen, /SS is not used for SPI communication because it is used to control the direction of the RS485 transceiver; you can use any digital I/O line as a /SS signal.

It is important to note that when the CPHA bit is 0, the /SS line must be de-asserted and re-asserted between each successive data byte exchange (M68HC11 Reference Manual, p.8-3). A mode fault occurs when the SPI senses that a multimaster conflict (MC68HC11F1 Technical Data Manual, p.8-7) exists on the network as explained above in connection with the /SS input. This allows the processor that is master to control the input /SS pins of other CPU’s, for example. If the /SS pin of the master is an output, it can be controlled independently of the SPI system. Pre-coded device drivers configure the SPI for a standard data format, and routines defined in this chapter make it easy to customize a data format and baud rate for your application. 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. These detailed signal descriptions and cable diagrams are presented to provide complete information for those who have special communications requirements and for those who wish to make their own application-specific communications cables. The communications is asynchronous because no synchronizing clock signal is transmitted along with the data.

The Serial 1 port can be configured for either RS232 or RS485 communications at up to 19200 baud. Although the RS232 protocol specifies functions for as many as 25 pins, each communications channel requires only three for simple serial interfaces: TxD1 (transmit data), RxD1 (receive data), and DGND (digital ground). Its simplest implementation requires only three wires: one to transmit serial data, a second to receive serial data, what is rs485 cable and a third to provide a common ground reference. A ground connection is also necessary to ensure that the communicating devices have a common voltage reference. You can use it to communicate with other devices. The program facilitates the development, debugging and analysis of software and hardware solutions that use serial communications for data exchange. The QScreen Controller, however, does not implement hardware handshaking. However, note that the functions that write to EEPROM disable interrupts for 20 msec. 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. The /SS (active-low slave select) is typically used to enable data transfers by slave devices when it is active low. If the /SS input to a slave is active (low), the slave transfers data in response to the SCK clock input that is initiated by the master.

In this example, the QVGA Controller selects the serial A/D by outputting a LOW signal on /SS. Rather, the UART deduces the correct time to sample the incoming signal based on the start and stop bits in the signal itself. Routines that temporarily disable interrupts for significant periods of time can also interfere with the Serial2 port. 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. Be sure to account for these effects when designing your application. If your application requires use of the secondary serial port as well as other interrupt routines, the key is to keep the interrupt service routines short and fast. The secondary channel is very useful for debugging application programs that communicate with other computers or I/O via the primary channel. 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.