HELUKABEL'S RS485 SIGNAL CABLES

The following table shows the connection diagram for a standard 9-pin serial cable. The QScreen Controller’s transmit data signal TxD1 (pin 2 on the 9-pin serial connector) is connected to the terminal’s receive data signal RxD (pin 2 on its 9-pin connector). To use a QScreen as a slave in a multi-drop network, simply define a word, (named Silence(void), for example) that when executed calls RS485Receive() to wait for any pending character transmission to complete, then disable the transmitter, and then execute a routine such as Key() to listen to the communications on the serial bus. RS-485 is also used in building automation as the simple bus wiring and long cable length is ideal for joining remote devices. With multi-drop (many devices on the same bus) and extensive cabling lengths, rs485 cable it's easy to see why the RS485 is so popular in the industrial and automation worlds. If your application requires RS485, you can use the secondary serial port (serial2) to program and debug your application code using the RS232 protocol, and use the primary serial port (Serial1) for RS485 communications.

Because all of the serial I/O routines on the QScreen Controller are revectorable, it is very easy to change the serial port in use without modifying any high level code. Control and monitoring systems: A single pair of rs485 signal cable can connect up to 32 devices, with a signal transmission distance of up to 1200m. Therefore, using rs485 cables is an ideal choice for industrial control and monitoring systems, especially those with large-scale and high dispersion. In a finished instrument, either or both channels can be used to communicate with other serial devices, or with other computers and/or terminals using RS232 or RS485. The RS485 data direction of Serial1 and Serial2 are controlled by the PJ0 and PJ1 processor pins, respectively. Although the RS232 protocol specifies functions for as many as 25 pins, each communications channel requires only three for simple serial interfaces: /TxD1 (transmit data), /RxD1 (receive data), and DGND (digital ground). Two RS485 transceivers are present on the PDQ Board, one for each channel. Since both channels can operate simultaneously and independently, serial debugging can be performed while the application program is communicating via its primary channel.

The advantage of using Serial1 for RS485 is that the Serial1 RS485 signals are also available on the Docking Panel, while the Serial2 RS485 signals are available only on the PDQ Board’s Serial Communications Header. The PDQ Board does not differentiate between these. The mating 10-pin connectors that join the H6 header of the PDQ Board to the H4 header of the Docking Panel are typically not accessed directly, and are not discussed in detail here. The PDQ Board, however, does not implement hardware handshaking. Many terminals and PCs, however, do rely on hardware handshaking to determine when the other party (in this case the PDQ Board) is ready to accept data. By connecting pairs of these handshaking signals together, the terminal or PC can be made to think that the PDQ Board is always ready to send and receive data. The RS232 protocol provides for four handshaking signals called ready to send (RTS), clear to send (CTS), data set ready (DSR), and data terminal ready (DTR) to coordinate the transfer of information. These protocols are summarized on this page, but for more information regarding their data formats and their use for simplex or multi-drop serial lines, consult Understanding Serial Communications (but keep in mind that that page is directed to the use of the UART Wildcard, so it uses different driver functions).

RS232 uses inverse logic; that is, a positive bit at the HCS12 UART is inverted by the onboard RS232 driver chip and appears as a negative signal on the serial cable. The Serial ports are implemented by the dual on-chip hardware UARTs (Universal Asynchronous Receiver/Transmitters) on the Freescale 9S12 (HCS12) microcontroller. The Serial1 and Serial2 ports have identical communications capabilities, although more of the Serial1 signals (both RS232 and RS485) are made available on the Docking Panels headers and connectors. The two values provide a sufficient margin for a reliable data transmission even under severe signal degradation across the cable and connectors. 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. From the PDQ Board’s point of view, these three signals (/TxD, /RxD, and ground) are the only connections required to perform serial communications.