Applications/Solutions/Technologies of RS485 to Ethernet

Date：2010-09-03 Author：Shanghai ZLAN Views：3574

RS485 communication technology is used a lot in engineering, its main advantages are as follows:
1). Simple wiring: only 2 lines are required, and shielded wire interface is generally used now.
2). Long communication distance: the communication distance of differential mode is farther than that of RS232.
3). Multi-device communication can be supported: the RS485 can mount multiple terminals on line and use the query-answer to realize multi-machine communication, compared with RS232 only one-to-one connection.

But RS485 now often needs to switch to Ethernet, as converting to TCP/IP, and the reasons behind these applications are:
1). Ethernet can be connected to the Internet so that long-distance communication can be realized, and monitoring is realized globally, which is impossible for RS485.
2). When multiple RS485 devices are required to connect to the computer, the traditional way is to use multiple serial port CARDS. It is used to extend the serial port on the computer to implement multiple RS485 serial ports. After using RS485 to Ethernet, the interface of RS485 can be saved to the scene, while the monitoring computer can only keep one network line, which is convenient to use.

To realize RS485 to Ethernet (as 485 to network port) there mainly have a hardware converter and a software driver. The hardware converter is divided into two kinds: serial device server (serial port networking server, serial port communication server) and serial port networking module.

To realize RS485 to Ethernet (as 485 to network port) there mainly have a hardware converter and a software driver. The hardware converter is divided into two kinds: serial device server (serial port networking server, serial port communication server) and serial port networking module.The RS485 serial device server is with shell, which connects the existing 485 devices to Ethernet. It is suitable for network transformation of existing RS485 serial port device. RS485 serial port networking module is without a shell, can be understood as the way of embedded RS485 to Ethernet module, using the module solution, the module can be integrated to the interior of the user device, realize the integration of serial device server and device. The price of module scheme is lower than the serial device server solution.

The hardware serial device server implements the RS485 port to TCP/IP interface (network interface), then used with the virtual serial port driver of computer can virtually convert the TCP/IP connection into serial port. The virtual serial driver can simulate virtual serial Numbers such as COM5 and COM6 in the drive layer in the computer, and bind the remote serial device server and these virtual serial ports. In this way, when the user serial port opens the virtual serial port, it can communicate with the remote serial device server, namely, communicate with the serial port device.

After using the RS485 to Ethernet solution for hardware and software systems, the 485 devices of user are equivalent to networked extensions, and the original hardware and software programs need not be modified.

RS485 wants to seamlessly upgrade to Ethernet, which contains a number of key technologies that require a seamless transition from Ethernet technology to RS485 communication protocols. These include: 9 technology, frame technology, master slave mechanism, etc.

9 bits: in 485 mode, users can send 9 bits of data. And sometimes we use the 9th bit to indicate whether it's a data frame or an address frame. Because Ethernet is 8bit data in network communication, there is no room for 9bit. So how to transfer the 9 bits of 485 is a problem. It is understood that the serial device server in Shanghai ZLAN is using the protocol called realcom, which can convert the 9 data format of RS485 into 8 bits of Ethernet.

Framing techniques: framing is a key technology in serial port to Ethernet (including the RS485 Ethernet), namely when a serial device server receives the data of the serial port device, when to forward the data to the Ethernet, if every byte hit for a package obviously the efficiency is too low, so how many bytes of packaging. This requires the user to set a parameter called the maximum packet length and the maximum packet interval.