Integrating serial based BMS systems into an Ethernet network
Most legacy BMS systems have communications interfaces based on RS232 or RS485 protocols. Examples of this include the proprietary ‘Satchnet’ protocol from Satchwell Controls and the open ‘Modbus’ standard originally developed by Modicon in 1978.
The Serial to Ethernet converter
The ‘serial to Ethernet converter’ has become a very popular communications gateway for network enabling serial based BMS equipment. This allows old hardware to be used in a modern Ethernet environment, potentially saving money on un-necessary hardware upgrades.
A serial to Ethernet converter can be applied in two distinct ways:
1. Virtual COM port
In this mode, a serial to Ethernet converter is deployed on an Ethernet network, providing network connectivity to a serial based outstation. Installed on the PC (potentially the supervisor / front-end) are COM port redirector drivers that take the serial data out of Ethernet frames and present the data as though it were from a serial port directly attached to the PC.
The drivers on the PC and the serial to Ethernet converter work in tandem, providing a path for serial data. At one end the presentation is a physical COM port on the converter, at the other end, the data emerges from a Windows COM port in software. To monitor the data, simply point your BMS software at the appropriate COM port.
2. Serial Tunneling
Serial tunnelling is a simple application of serial to Ethernet converters used in pairs to provide a ‘serial tunnel’ through an IP network. What goes in one end comes out the other and this can save the cost of running 100s of metres of serial cabling by using the already installed Ethernet network for data transmission. In some cases, IT departments can be sensitive about access to their network by third parties. This problem has been solved previously by setting up a dedicated VLAN* for the BMS traffic. Alternatively, the site may have un-used copper or fibre optic cables, in which case the addition of industrial Ethernet switches will provide a dedicated BMS network.
Because the serial to Ethernet converter operates over any TCP / IP network, a LAN, WAN or even the internet can be used to send data around the world. Serial data can easily be shared from site to site using an existing WAN or by the addition of internet connectivity.
Versions of serial to Ethernet gateways dedicated to the conversion of Modbus serial to Modbus/TCP (Ethernet based Modbus) are also available. These devices have additional capabilities to ensure that complete messages are transmitted in line with the requirements of the Modbus protocol.