If you need more information on the cable & protocol, refer to the article below. So after an hour of digging through what little information I could find, I decided to look at the 900mhz modems they are using to backtrace any information. In these distributed processing networks, a variety of machines and instruments work locally, but communicate and share data or resources with one another globally using a single serial link. The SCK pin’s synchronous clock signal has configurable phase, polarity and baud rate so that it can interface to a variety of synchronous serial devices. It provides a convenient means of connecting the QScreen Controller to a variety of peripheral devices, including analog to digital and digital to analog converters, real time clocks, and other computers which use high speed communication. The RS485 communication interface allows the slave unit (i.e. control module) to be interrogated and some options programmed by a remote computer.
The symmetrical pin layout of the RJ11 sockets on the Unitronics PLCs allows an easy way of interconnecting the PLCs with both RS232 and RS485 communications. If you are using the QScreen as a slave device and require the /SS signal for your external SPI hardware, configure one of the Port A pins on the Field Header as an input pin. By polling the Port A pin or by setting up an interrupt service routine, you can configure the QScreen to ignore the SCK input when /SS is high and keep MISO in a high-impedance state so that it does not interfere with the SPI bus. Note that the master device outputs the clock synchronization signal SCK to the slave’s SCK which is configured as an input. If the /SS pin of the master is an input and if a low input level is detected, the processor sets the MODF bit in the SPI status register a "mode fault" condition. When the /SS input goes low, the slave (or QScreen in this case) transfers data in response to the SCK clock input that is initiated by the master.
This automatically activates the SCK clock which synchronously transmits the data. The master and slave can then exchange data. 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. Finally check the documentation on your RS485 device to see how to set it up, and if can be disabled by default. QED-Forth includes three built-in routines to facilitate control of the RS485 transceiver. When this bit is high, the transceiver is in transmit mode. The QScreen Controller controls the RS485 transceiver with bit 5 of Port D of the processor. For the QScreen, /SS is not used for SPI communication because it is used to control the direction of the RS485 transceiver; you can use any digital I/O line as a /SS signal. This ability to exchange messages means that the SPI is capable of full duplex communication. With Modbus RTU, sending and receiving are never simultaneous, so half duplex is as fast as full duplex. Slave devices use the master in/slave out pin, MISO, for transmitting, and the master out/slave in pin, MOSI, for receiving data.
The byte-sized messages are transmitted and received via the MOSI (master out/slave in) and MISO (master in/slave out) pins. Transmissions are always initiated by the master device, and consist of an exchange of bytes. From what I’ve read, the modbus is a protocol for communication between a computer and an RS485 device, if this is true, I shouldn’t need a modbus lib for this application. RS485 cable is a type of communication cable commonly used in industrial settings for connecting devices in a network. When the network master wants to talk to this particular slave, it outputs the slave’s ascii name onto the serial bus. When the keyword name is received by the Silence() routine running in the slave, the slave QScreen Controller executes RS485Transmit() to send an acknowledgment to the master (which should now be listening to the serial bus to accept the acknowledgment). There are many applications of RS485 in weak current intelligent engineering, and the most common applications are in access control systems, BA systems, intelligent lighting systems, parking lot management systems, etc. With the continuous popularization of digital networks, RS485 is now less and less used in weak current applications.
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