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. 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. 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. By polling the Port A pin or by setting up an interrupt service routine, you can configure the QScreen to ignore the SCK input when /SS is high and keep MISO in a high-impedance state so that it does not interfere with the SPI bus. 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.

This ability to exchange messages means that the SPI is capable of full duplex communication. The master and slave can then exchange data. In the most common multi-drop RS-485 protocol, one computer is designated as a "master" and the rest of the computers or devices on the serial bus are designated as "slaves". The interface can be used to support analog to digital and digital to analog converters, networks of many computers controlled by a single master, or networks of devices controlled by several coordinated masters. Connecting computers together in multi-drop networks is common in factories and laboratories. This section describes the QED-Forth routines that control the RS485 transceiver, and presents some ideas that may prove useful in designing a multi-drop data exchange protocol. This chapter describes those drivers, and presents code that makes it easy to configure the SPI for different data transfer rates and formats. The Serial2 channel is always configured for RS232 communications, rs485 cable and can sustain baud rates up to 4800 baud. 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 words port and channel are used interchangeably to refer to a serial communications link.

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. Pre-coded device drivers configure the SPI for a standard data format, and it is easy to customize a data format and baud rate for your application. The QED-Forth kernel includes pre-coded drivers that configure and control the SPI for maximum speed data transfers. It provides a convenient means of connecting the QScreen 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. Limitless Genuine Hybrid DVR for HD-SDI and Analog. HD-SDI series delivers crystal clear mega pixel images without compression over coax. Care must be taken that an SC connection, especially over long cable runs, does not result in an attempt to connect disparate grounds together - it is wise to add some current limiting to the SC connection.

Another advantage over RS232 that I mentioned earlier is the number of connectable devices. The device that initiates a data transfer is the master, and all other devices on the network are slaves. 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. Grounds between buildings may vary by a small voltage, but with very low impedance and hence the possibility of catastrophic currents - enough to melt signal cables, PCB traces, and transceiver devices. Two wire telephone cables will not function because in that case the ground signal levels on both sides will be floating. This signal synchronizes the exchange of bytes between the QScreen and its peripherals. The master and slave could even exchange ascii QED-Forth commands. When the network master wants to talk to this particular slave, it outputs the slave’s ascii name onto the serial bus.