Interface RS485
The RS485 interface is quite common in fieldbus networks. This high-speed and interference-resistant serial interface makes it possible to network many devices via a common bus line.
The data are transferred via two wires (D+ and D-) in form of a "symmetrical" (differential). The maximal distance between the extreme devices may reach 1200 m. With an RS485 amplifier the maximum distance can be even greater. If the communication line is 100 m or longer, it is strongly recommended to use termination resistors (from 100 to 250 ohm) at both ends of the line to eliminate reflections. The number of devices in any RS485 network may not exceed 32 nodes (without repeaters).
Typical fieldbus network based on the RS485 interface
A usual office PC is not equipped with the RS485 interface compared to an industrial PC. For this reason, an interface adapter is required to connect a PC to a fieldbus network. For this purpose, akYtec offers USB-RS485 interface converter IC4 and RS232-RS485 interface converter IC3
Interface RS232
This interface is based on EIA RS232C and is intended for a serial connection between two devices. It is often used to connect external equipment to a PC. The RS232 interface implements an asynchronous serial communication form of signal transmission over two lines, TxD (transmit data) and RxD (receive data), while the signal amplitude is measured relative to the grounding line (GND).
Since data transmission over the RS232 interface is normally asynchronous, there is a low interference immunity and thus the route length is limited to a few meters. Two separate communication lines allow full-duplex data transmission, meaning reception and transmission of data happen simultaneously.
The RS232C specification does not contain a description of the connector to be used. However, the most commonly used type is the 9-pin and 25-pin connectors (DB9 and DB25).
Ethernet
Ethernet is a data transfer technology for connecting devices to computer networks, usually to local area networks (LAN). Data access in these networks is controlled by the CSMA/CD algorithm. This acronym stands for Carrier-Sense Multiple Access with Collision Detection. In this method, devices only exchange signals when they have detected an idle state on a wire in order to avoid collisions. All forms of Ethernet support up to 1024 nodes in a network. This interface is widely used due to its high data bandwidth and immunity to interference.
Interface USB
Standard USB was developed as an alternative to the previous network standards RS232 and IEEE 1284 to connect peripheral devices to PCs. Currently, many devices are equipped with the interface USB 2.0, so that data can be transmitted at the ate up to 480 Mbps. The USB interface as well as the RS485 interface can transfer the data over two wires (D+ and D-) and the logical levels are identical to those of the RS485 standard. This interface has two additional wires (Vcc and GND) for supplying power to the connected device (provided that its current consumption is 500 mA max.).
Once the appropriate driver is installed, the OS automatically recognizes the device connected to the USB port as a COM port and operates in the standard asynchronous transfer mode, which is used to operate with a hardware COM port.
Asynchronous data transmission
The RS232 and RS485 interfaces support asynchronous data transmission. The data are transmitted byte to byte. To signify the beginning and end of each byte (character) to be transmitted, each byte is framed on both sides by a start and a stop bit. The bits between the start and the stop bits are called data bits. They can be 5, 6, 7, or 8 bits. The number of standard data transfer rates have been established for asynchronous transmission: 50, 75, 110, 150, 300, 600, 1200, 2400, 4800, 9600, 19200, 38400, 57600, 115200 bit/s.
Concerning the protocols, most of akYtec devices communicate via Modbus ASCII/RTU. Further details will be published in one of our next articles.