1 or 2 to specify Serial1 or Serial2, respectively, and clears the appropriate PORTJ bit to place the transceiver in receive mode. 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. 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). 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. 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. Without termination resistors, reflections of fast driver edges can cause multiple data edges that can cause data corruption. To solve the "data collision" problem often present in multi-drop networks hardware units (converters, repeaters, micro-processor controls) can be constructed to remain in a receive mode until they are ready to transmit data.

Two RS485 transceivers are present on the PDQ Board, one for each channel. In general if you are not connected to a modem the handshaking lines can present a lot of problems if not disabled in software or accounted for in the hardware (loop-back or pulled-up). With the introduction of "automatic" repeaters and high-impedance drivers / receivers this "limitation" can be extended to hundreds (or even thousands) of nodes on a network. RS422 devices. These networks are often used in a half-duplex mode, where a single master in a system sends a command to one of several "slave" devices on a network. In order to avoid errors when many devices are connected, cables of the same colour should be used for all the connections to the terminals A and cables of the same colour should be used for all the connections to the terminals B of the various devices (e.g. white for A and blue for B). There are modules for RS323-DCE (DB9F), RS232-DTE (DB9M), and RS485 interfaces.

Receiver Input Resistance (Ohms), (1 Standard Load for RS485) 3k to 7k 4k min. EIA-485 only specifies electrical characteristics of the driver and the receiver. 12 V and -7 volts will allow correct operation of the receiver. A jumper, J3, configures the primary serial port for either RS232 or RS485 operation. RS423 is another single ended specification with enhanced operation over RS232; however, it has not been widely used in the industry. Single master systems (many other communications schemes are available) offer a straight forward and simple means of avoiding "data collisions" in a typical 2-wire, half-duplex, multi-drop system. The PDQ Single Board Computer (SBC) has two asynchronous serial communications ports named Serial1 and Serial2. Two asynchronous communications ports named Serial1 and Serial2 can each be configured for RS232 or RS485 protocols. Serial data communications standards - RS-232, RS-422, & RS-485 were widely used for data transfer providing easy connectivity for the time and they can still be found today. In contrast to RS-422, which has a driver circuit which cannot be switched off, RS-485 drivers use three-state logic allowing individual transmitters to be deactivated. In a computer system, rs485 cable SCSI-2 and SCSI-3 may use this specification to implement the physical layer for data transmission between a controller and a disk drive.

EIA-485 (formerly RS-485 or RS485) is an OSI Model physical layer electrical specification of a two-wire, half-duplex, multipoint serial connection. This robustness is the main reason why RS-485 is well suited for long-distance networking in noisy environment. 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". While a multi-drop "type" application has many desirable advantages, RS422 devices cannot be used to construct a truly multi-point network. You may use nonstandard baud rates if both devices support them. If your application requires RS485, use the primary serial port (serial1) for RS485 communications, and use the secondary serial port (Serial 2) to program and debug your application code using the RS232 protocol. RS232 (single-ended) was introduced in 1962, and despite rumors for its early demise, has remained widely used through the industry. When communicating at high data rates, or over long distances in real world environments, single-ended methods are often inadequate. The Serial1 and Serial2 ports are is supported by the HCS12's dual on-chip hardware UARTs, and do not require interrupts to work properly. It is supported by virtually all personal computers, and is the default protocol for both of the QScreen Controller’s serial ports.