These factors are discussed in more detail in The UART Wildcard: Modem Handshaking Signals. The first of the RS standards was RS232, or more correctly RS-232. Each of the two channels on the UART Wildcard can be configured for RS232, RS422, or RS485. Each of the two channels on the UART Wildcard implements two 16-character FIFOs, one for outgoing characters and one for incoming characters. In RS485 mode, the RS422 transmit and receive pairs are shorted together with a pair of onboard jumpers as explained in the UART Wildcard Hardware: RS485 Jumpers section below. A mode fault occurs when the SPI senses that a multimaster conflict (MC68HC11F1 Technical Data Manual, rs485 cable p.8-7) exists on the network as explained above in connection with the /SS input. The data bits are also transmitted in reverse order, with the least significant bit transmitted first, after a start bit. 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. The RS232 driver and receiver use separate conductors on the serial cables, enabling full duplex communications. Note that the local and the remote must share a common ground, so a minimum of 3 wires are required for half duplex RS485 communications: a pair of transceive wires and a common ground.

Note that the local and the remote must share a common ground, so a minimum of 5 wires are required for full duplex RS422 communications: two transmit wires, two receive wires, and a common ground. The most widely used protocol is RS232, a full duplex protocol with a single-ended bipolar voltage swing on the serial cable. 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. Many terminals and PCs, however, do rely on hardware handshaking to determine when the other party (in this case the QScreen Controller) is ready to accept data. Owing to hardware constraints, if modem handshaking is needed on UART channel 1, then channel 1 must be configured for RS232, and channel 2 cannot be configured for RS232 communications. A modem (modulator/demodulator) provides a way of encoding digital data as a set of audio signals that can be sent over a telephone line. The UART Wildcard implements these optional RS232 modem handshaking signals on channel 1. The handshaking signals can be disabled and/or ignored by applications that do not need them. 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.

Each RS422 driver transmits a differential pair of output signals at 0 and 5 volts. Each RS232 driver uses inverting logic and implements a single-ended bipolar output voltage (that is, one signal that swings above and below ground). 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. The Serial 1 port is implemented with the 68HC11's on-chip hardware UART (Universal Asynchronous Receiver/Transmitter). Most modems communicate using RS232 and a set of hardware handshaking signals used to regulate data flow. The received data byte is accessed by reading SPDR data register. WCOL is cleared by a read to the SPSR followed by a read or write to the SPDR. Thus RS485 is the standard protocol of choice when multi-drop communications are required. The RS485 protocol uses differential data signals for improved noise immunity; thus RS485 can communicate over greater distances than RS232. The serial data stream at the UART is conditioned by serial driver chips that transmit and receive the data.

In fact, a single driver chip on the UART Wildcard is used to implement both RS422 and RS485 communications for a given serial channel. 5 Volt supply by a dual RS232 driver chip that has a built-in charge pump voltage multiplier. The terminal’s serial receiver chip re-inverts the signal to its positive sense. The RS422 receiver converts the differential signal to the 0 to 5 volt logic signal required by the UART circuitry. Because a mark (logic 1) condition is traditionally represented (e.g. in RS-232) with a negative voltage and space (logic 0) represented with a positive one, A may be considered the non-inverting signal and B as inverting. However this contradicts the facts that an idle mark state is a logical one and the termination polarization puts B at a higher voltage in Profibus guidelines. In order to avoid signal reflections, a 120 Ohm termination resistance must be fitted on each end of the main cable. Normally, this connection is made at one end of the main cable. The end resistance must be used only at the ends of the main cable. The resulting signal levels on the interface cable connect the local and remote in a manner specified by a standard protocol.