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The RS485 system used for Modbus communication provides a main cable (Bus or backbone), to which all the devices have to be connected with branches (also known as stubs) that are as short as possible. Ideally, the two ends of the cable will have a termination resistor connected across the two wires. Star and ring topologies are not recommended because of signal reflections or excessively low or high termination impedance. If a star configuration is unavoidable, special RS-485 repeaters are available which bidirectionally listen for data on each span and then retransmit the data onto all other spans. Since RS-485 is a multi-point specification, however, this is not necessary or desirable in many cases. While a multi-drop "type" application has many desirable advantages, RS422 devices cannot be used to construct a truly multi-point network. 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. They translate the bit-by-bit data on the serial cable into bytes of data that can be interpreted by the operating system or by your application program.

Without termination resistors, signal reflections off the unterminated end of the cable can cause data corruption. 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. RS-485, like RS-422, can be made full-duplex by using four wires. RS-485, also known as TIA-485(-A) or EIA-485, is a standard, originally introduced in 1983, defining the electrical characteristics of drivers and receivers for use in serial communications systems. 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. This allows RS-485 to implement linear bus topologies using only two wires. The RS485 network is wrongly referred to as a "2 wires plus shield" network.

The shield works as a "drain" for any noise that could be picked up by the RS485 network and "drain" it to ground. No matter the devices, it is always good on real site installation to use a shielded cable, with 18-22AWG wiring and the shield connected ONLY to one end of the line. The device ships with an Ethernet RS-485 communication interface as well as a RS-485 to USB cable, which is said to be able to communicate with a PC using proprietary Windows based software (and driver, I think). These characteristics make RS-485 useful in industrial control systems and similar applications. 2 wires is all you need to make it work. You need a special Ethernet cable for realizing this communication protocol. Entire USB protocol handled by the electronics in the cable. The default serial routines used to download programs to the operating system assume that full duplex communications are available, so you cannot use the RS485 protocol to program the controller. The Serial 2 port is dedicated to RS232 communications at up to 4800 baud. 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.

From the PDQ Board’s point of view, these three signals (/TxD, /RxD, and ground) are the only connections required to perform serial communications. There are surface-mount resistor pads on the Docking Panel to bring out the RS485 signals to the DB9 Serial 1 Connector. The RS232 signals are represented by voltage levels with respect to a system common (power / logic ground). RS422 (differential) was designed for greater distances and higher Baud rates than RS232. 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. Since it uses a differential line over twisted pair (like EIA-422), it can span relatively large distances (up to 4000 feet or just over 1200 metres). It does not specify or recommend any communications protocol; Other standards define the protocols for communication over an RS-485 link. Digital communications networks implementing the standard can be used effectively over long distances and in electrically noisy environments. With noisy environments (our sites?) using the shield as ground is not always the most effective solution. This allows for basic error detection, in that if noise on the transmission line causes one bit to be received incorrectly, either received as a '0' when transmitted as a '1' or vice-versa, the error would be detected due to the count of '1' bits in the byte being odd when it is expected to be even, or vice-versa depending on the parity checking settings.

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