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Most computers conform to IBM PC AT-compatible RS232 interfaces which use 9-pin D-Type connectors, consequently the QScreen Controller brings out its serial ports to two female 9-pin D-Type connectors. Connecting computers together in multi-drop networks is common in factories and laboratories. You can use the QScreen’s RS485 link to create such a multi-drop serial network. The existence of an RS485 port on the socket makes it even a little bit more difficult. If more than one slave tried to drive the transmit line simultaneously, their serial drivers would fight with each other for control of the bus. The board with dual RJ11 jacks is the more popular since is can use commercial straight-through, non-reversing, 6-wire data cables with RJ11 connectors. The RJ11 socket in the Unitronics PLCs provides six signal pins. The RJ11 socket is interesting, because the manufacturer choose to have a symmetrical pin-layout, just as with the MMJ connector on DECconnect systems and the RJ45 jack in the Yost standard. 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. Two wire telephone cables will not function because in that case the ground signal levels on both sides will be floating.

If a slave device has already stored a byte into its SPDR register, that byte will be exchanged with the master’s byte. After a data transfer is initiated by writing to the SPDR data register, the processor may poll the SPSR status register until the SPIF flag is set. 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. 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. 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. This section also defines the logic states 1 (off) and 0 (on), by the polarity between A and B terminals.

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 contrast to EIA-485 (which is multi-point instead of multi-drop), EIA-422/V.11 does not allow multiple drivers but only multiple receivers. 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. 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 default serial routines used by the onboard kernel assume that full duplex communications are available, so you cannot use the RS485 protocol to program the controller. 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. 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.

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. Typing a carriage return at the terminal should now produce the familiar "ok" response via the Serial2 port. The maximum Serial2 communications rate is 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. 00-3) or standard 12V power supplies. The EIA once labeled all its standards with the prefix "RS" (Recommended Standard), but the EIA-TIA officially replaced "RS" with "EIA/TIA" to help identify the origin of its standards. We’re eager to help. The QScreen Controller’s kernel software contains a complete set of high level driver routines for the Serial2 port, and these functions are summarized in the Control-C Glossary. 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. Function prototypes for this function and other versatile serial I/O routines are defined in the COMM.H header file, and are described in detail in the Control-C Glossary.

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