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For those of you interested in the details, here’s how it works: The low-level serial driver routines named Key(), AskKey() and Emit() are revectorable routines that can be redirected to use either of the serial ports. These signals may alternatively be redirected to the digital inputs and outputs used by the second serial port if hardware handshaking is required. We recommend that you keep the faster Serial1 port as the default serial link as you work through the exercises in this book. 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. Because all of the serial I/O routines on the QScreen Controller are revectorable, it is very easy to change the serial port in use without modifying any high level code.

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. The GROUND line serves as a common voltage reference for the master and slave. If more than one slave tried to drive the transmit line simultaneously, their serial drivers would fight with each other for control of the bus. The master and slave could even exchange ascii QED-Forth commands. At any given time, only the master and a single active slave communicate. The /SS (active-low slave select) is typically used to enable data transfers by slave devices when it is active low. In this example, the QScreen Controller selects the serial A/D by outputting a LOW signal on /SS. 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. You can use the QScreen’s RS485 link to create such a multi-drop serial network. To use a QScreen as a slave in a multi-drop network, simply define a word, (named Silence(void), for example) that when executed calls RS485Receive() to wait for any pending character transmission to complete, then disable the transmitter, and then execute a routine such as Key() to listen to the communications on the serial bus.

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. A data transfer is initiated by a master device when it stores a message byte into its SPDR register. When the QScreen controls the network, it is referred to as a "master"; otherwise, it is a "slave". The DWOM bit determines whether Port D needs pull-up resistors; it should be set to 0. The MSTR bit determines whether the device is a master or slave. The byte-sized messages are transmitted and received via the MOSI (master out/slave in) and MISO (master in/slave out) pins. Thus RS485 is the standard protocol of choice when multi-drop communications are required. The communications is asynchronous because no synchronizing clock signal is transmitted along with the data. Rather, the UART deduces the correct time to sample the incoming signal based on the start and stop bits in the signal itself. The secondary serial port is implemented by a software UART that controls two pins on PortA. 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.

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. The MODF bit is cleared by a read of the SPSR followed by a write to the SPCR. The SPIF is set when a data transfer is complete, and is cleared by a read of the SPSR status register, followed by a read or write to the SPDR data register. Given a properly wired network and a properly configured SPCR control register, a master device may transmit a message by simply storing the byte to the SPDR data register. The two lowest order bits in the SPCR control register, named SPR1 and SPR0, determine the data exchange frequency expressed in bits per second; this frequency is also known as the baud rate. You can operate the port at any baud rate up to 4800 baud; just specify the rate you want before the BAUD2 command.

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