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To avoid contention on the RS485 bus, the application software must assure that only one transmitter is enabled at a time. These steps greatly reduce the chance that the communicating devices might be damaged by contention on the SPI bus. Rather, it relies on software handshaking via transmission of XON/XOFF characters to coordinate data transfer and ensure that information is not lost when one of the communicating parties is busy. Similarly, when establishing the transmit mode, the application software is responsible for ensuring that the RS485 receiver is not disabled until all expected characters have been received. The master can instruct a single slave to go into transmit mode, and then the master can put itself into receive mode, thereby allowing the master to retrieve data from the slave. Care must be taken when changing an RS485 node from transmit mode to receive mode, or visa versa. The shield works as a "drain" for any noise that could be picked up by the RS485 network and "drain" it to ground. The RS485 protocol uses differential data signals for improved noise immunity; thus RS485 can communicate over greater distances than RS232.

The most common cable type is Cat 5e-UTP (unshielded twisted pair) which may work over shorter distances in less demanding applications with low EMI noise levels. RS232 is by far the most common protocol. RS232 allows both communicating parties to transmit and receive data at the same time; this is referred to as full duplex communications. Unlike the standard RS232 protocol, RS485 allows many communicating parties to share the same 3-wire communications cable. Since both channels can operate simultaneously and independently, debugging can be performed while the application program is communicating via its primary channel. In this manner, data can be exchanged between the master and each slave on the bus. 989226-01M. Inter-Module/Module Master Controller Cable. The QScreen Controller does not differentiate between these. We can gain insight into the operation of the RS232 protocol by examining the signal connections used for the primary serial port in Table 9 6. The transmit and receive data signals carry the messages being communicated between the QScreen Controller and the PC or terminal. 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. The primary and secondary serial communications ports are accessible through the QScreen's 10 pin, dual row Serial Header (H5) which is typically not installed, the 24 pin, dual row Field Header (H3), and through the individual DB-9 Serial 1 and Serial 2 connectors. RS232 uses inverse logic; that is, a positive bit at the 68HC11 UART is inverted by the onboard RS232 driver chip and appears as a negative signal on the serial cable. The data bits are also transmitted in reverse order, with the least significant bit transmitted first, after a start bit. Like RS232, the data bits are transmitted in reverse order, with the least significant bit transmitted first. Rather, the UART deduces the correct time to sample the incoming signal based on the start and stop bits in the signal itself.

The Serial 1 port is implemented with the 68HC11's on-chip hardware UART (Universal Asynchronous Receiver/Transmitter). 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. The QScreen Controller has two serial communications ports: a primary serial port called Serial 1 that supports both RS232 and RS485 protocols, and a secondary serial port called Serial 2 that supports RS232. You can operate the port at any baud rate up to 4800 baud; just specify the rate you want before the BAUD2 command. RS232’s greatest benefit is its universality; practically all personal computers can use this protocol to send and receive serial data. It is supported by virtually all personal computers, and is the default protocol for both of the QScreen Controller’s serial ports. There are several protocols that govern the format of exchanged data, with the RS232 protocol used primarily by personal computers, and the RS485 protocol used in industrial control systems. In theatre and performance venues, RS-485 networks are used to control lighting and other systems using the DMX512 protocol.

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