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The QScreen Controller controls the RS485 transceiver with bit 5 of Port D of the processor. After a data transfer is initiated by writing to the SPDR data register, the processor may poll the SPSR status register until the SPIF flag is set. Then use front panel programming to set the baud rate, protocol, digital address, half or full duplex, and other available parameters. In all other cases, half duplex and full duplex will work equally well. Thus, as a rough approximation, operating at 4800 baud full duplex requires about 40 to 50% of the 6811's CPU time (that is, an average of approximately 40 to 50 µs service time every 100 µs). This ability to exchange messages means that the SPI is capable of full duplex communication. 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 QScreen allows the details of the synchronous communications protocol to be customized for compatibility with a variety of peripherals. This function cannot accept incoming data; consult its glossary entry for details. Consult their glossary entries for details. The Control-C Glossary contains a list of functions that temporarily disable interrupts, and the glossary entries give further information regarding how long interrupts are disabled. 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. You might also consider operating the secondary serial port at a lower baud rate to relax the timing constraints. 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. Fast auto turn-around, from transmit to receive mode, requires no software delays and eliminates timing problems and data collisions. Because the requirements of every multi-drop application are so unique, it is difficult to specify or design a software protocol that meets everyone’s needs. Connecting computers together in multi-drop networks is common in factories and laboratories. The one you choose depends on the specific device, or devices you will be connecting to.

Transmissions are always initiated by the master device, and consist of an exchange of bytes. When the exchange is complete, the slave can again execute the Silence() routine to disable its transmitter and begin listening for its name. When the network master wants to talk to this particular slave, it outputs the slave’s ascii name onto the serial bus. When the keyword name is received by the Silence() routine running in the slave, the slave QScreen Controller executes RS485Transmit() to send an acknowledgment to the master (which should now be listening to the serial bus to accept the acknowledgment). If you are running Serial2 at 4800 baud, what is rs485 cable the rest of your application must be able to function properly using the remaining portion of the CPU time. The status of a device as master or slave determines how the various pins must be configured. Any required SPI output signals must be configured as outputs, either by calling InitSPI() or by setting the appropriate bits in the Port D data direction register DDRD. Hardware is interfaced to the SPI via three PORTD pins named SCK, MOSI, and MISO brought out to pins 7, 8, and 10 on the Wildcard Port Header (see Appendix B).

The arrows in the diagram point to pins configured as inputs, and originate from output pins. If the programmer has enabled the local interrupt mask for the SPI, an interrupt is recognized at this point. Alternatively, the if the SPI interrupts are enabled, the SPI interrupt handler determines what caused the interrupt by reading the SPSR register to see which of the three status bits is set. 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. However, note that the functions that write to EEPROM disable interrupts for 20 msec. We have built sophisticated instruments using the QScreen Controller that operate very reliably using multiple interrupts in addition to the software UART. Although data byte transfers are easily executed once the network has been wired and configured properly, a carefully executed software protocol may be required to ensure data integrity. The device that initiates a data transfer is the master, and all other devices on the network are slaves. It may be that only the byte sent from the master to the slave is meaningful; nevertheless, each device simultaneously transmits and receives one byte.