By no means Lose Your Rs485 Cable Again

It is important to know how to wire devices on an RS485 network, especially when a non opto-isolated device is present. Combined with the right cables, they are going to guarantee the creation of stable and functional RS485 networks. The RS422 driver and receiver use separate differential conductor pairs on the serial cables, enabling full duplex communications. In its simplest form, a pair of converters from RS232 to RS422 (and back again) can be used to form an "RS232 extension cord." Data rates of up to 100K bits / second and distances up to 4000 Ft. The specification allows for data transmission from one transmitter to one receiver at relatively slow data rates (up to 20K bits/second) and short distances (up to 50Ft. @ the maximum data rate). Any number of characters can be sent, and the transmitter will automatically re-trigger with each new character (or in many cases a "bit-oriented" timing scheme is used in conjunction with network biasing for fully automatic operation, including any Baud rate and/or any communications specification, eg. If two bits are received incorrectly, the error will go unnoticed by parity checking.

Ideally, the two ends of the cable will have a termination resistor connected across the two wires. End of line termination resistor (120 ohm 1/4 watt) to be installed at both ends of network. The value of each termination resistor should be equal to the cable characteristic impedance (typically, 120 ohms for twisted pairs). Hah, for five years from now I wanted to repurpose the RJ10 phone modem connector on my Dell notebook with a mini PCIe CAN/RS485 card and some extra cable. Port to modem communications usually use 300, 1200, 2400, 4800, 9600, 19200, 38400, 57600 and 115200 baud. Software-selectable baud rates up to 56,000 baud are supported. Data transfer rates from 300 baud to 3 Mbaud. RS422 (differential) was designed for greater distances and higher Baud rates than RS232. RS485 is a half duplex version of RS422 that is capable of multi-drop operation. The ideal RS485 network requires a dedicated wire for ground.

But these are signal wires and the network always requires a return path. 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). The USB-RS485-PCBA converter provides a USB to RS485 serial interface with a simple USB type A connector and PCB for easy integration into a design or as the basis of a converter cable. UART interface support for 7 or 8 data bits, 1 or 2 stop bits and odd / even / mark / space / no parity. A break sequence forces the serial output to a logic low (space) at the UART. By setting this output LOW, the slave’s input /SS is pulled LOW. 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. If you have not yet compiled the GETSTART program and you want to do the exercises here, open GETSTART.C in your TextPad editor, click on the Make Tool, and after the compilation is done, enter Mosaic Terminal by clicking on the terminal icon and use the "Send File" menu item to send GETSTART.DLF to the QScreen Controller.

RS485 is another protocol supported by the primary serial port on the QScreen Controller. How do I wire RS485 devices? 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 many categories of Ethernet cables we can use like CAT-4, CAT-5, CAT-5E, CAT-6, CAT-6A, etc. In our tutorial, we are going to use CAT-6E cable which has 4 twisted pairs of 24AWG wires and can support up to 600MHz. It is terminated at both ends by an RJ45 connector. Entire USB protocol handled by the electronics in the cable. Visual indication of Tx and Rx traffic via LEDs in the transparent USB connector.

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