Ethernet to Serial Converters

VTScada supports the use of Ethernet to serial converters, also know as terminal servers, to allow connections from VTScada servers to serial I/O devices.

 

These provide a viable option to RS-232 cards, which are increasingly difficult to find. Experience indicates that USB to Serial converters may be unreliable. But, the most significant reason to consider a converter is redundancy. Unless the remote device has two serial ports, only one computer can be connected to it at a time. If the VTScada computer making that connection goes down, there is no failover until the second computer is connected.

With an Ethernet to serial converter however, the serial side can be connected to the device and the Ethernet side to a switch. VTScada can then be configured with a TCP port and any VTScada computer can become a redundant server for the IO.

 

Most of our drivers that support serial protocols can be used with these devices if you use the following configuration. (This list is the minimum required configuration. More steps may be required in some cases.)

 

  • Protocol mode set to "Raw"
  • Device listens for a connection from VTScada on a specific TCP or for packets on a UDP port
  • Device does not initiate the connection to VTScada
  • Device’s serial port settings (baud, parity, start & stop bits) are configured to match the I/O device
  • "Inactivity Time" automatic disconnect in the device set to a longer time period than the VTScada driver’s "Timeout" setting

 

After the driver is configured, use a TCP/IP or UDP/IP port tag with the appropriate VTScada serial driver to communicate through the device to your serial I/O device. If you are experiencing issues with communication when using the device that you do not see when connected directly using a serial port on your server, there could be problems with the device’s configuration. Common problems include:

 

  • The data packing scheme used by the device is based on it receiving specific characters that may not be present in the serial protocol.

Some devices allow multiple TCP/IP connections from multiple servers (VTScada and others). These setups work well with simple protocols that involve simple command response messaging (such as Modbus) but can cause problems with protocols that use more complex messaging schemes.

  • Devices sometimes "escape" data received on the serial port before forwarding to the Ethernet.

When this happens, specific characters received once on the serial port are doubled when the messages are sent on Ethernet. These typically show up as CRC or checksum errors in the VTScada drivers and can be difficult to diagnose without a good knowledge of the protocol details. The most common of these are on characters 0h10 (the ASCII DLE (Data Link Escape) character) and 0hFF. Note that many protocols also "escape" the ASCII DLE character, making this problem difficult to diagnose.

 

If you are having problems when using an Ethernet to serial converter that you don not have when connected directly via serial, search first for known issues with the device you are using.