Serial to TCP/IP converter software: COM port as TCP or UDP

A serial to TCP/IP converter makes a device on a COM port reachable at an IP address and a port number. TCP COM Bridge does this in software on a Windows computer: it opens the COM port, listens on a TCP port, and copies the bytes in both directions without changing them. Any program that can open a TCP connection can then talk to the device. A program that knows only COM ports needs a virtual COM port on its side as well.

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Set up the computer that owns the device

Add a connection. Start TCP COM Bridge and choose "Devices → Add new" (Ctrl+N).
Select the COM port. On the "Port" tab choose the COM port of the device and leave "Create virtual serial port" unchecked. Set the baud rate, data bits, parity and stop bits to the values of the device. Flow control is on the "COM flow control" sub-tab. For an RS232-RS485 converter that needs the RTS line switched during transmission, check "RS485 interface mode".
Fig. 1. The serial side: port number and communication parameters
Choose the network role. On the "IP settings" tab keep the protocol "TCP/IP" and select "Server (others will be connecting to this PC)". Enter the IP address of the network adapter that should listen, or 0.0.0.0 to listen on all adapters, and a port number above 1000, for example 10232.
Fig. 2. The network side: protocol, mode, address and port
Decide who may connect and in which direction. On the "Server mode" tab:
- "Max clients" limits simultaneous clients; 0 means no limit.
- "IP → COM" and "COM → IP" set the data direction. Block "IP → COM" when clients should only read the device.
- "Allowed IP addresses" takes one address per line. Connections from other addresses are rejected. An empty list allows everyone.
Fig. 3. Server options: number of clients, direction, allowed addresses
Save. Click "OK". Allow the program in the Windows firewall when Windows asks. The connection appears in the list of the main window with its state and byte counters.

Test it with a plain TCP client

You do not need a second copy of the program for a test. On another computer open a terminal program that can make a raw TCP connection, for example PuTTY with the connection type "Raw", and connect to the IP address and port of the server. Data from the device should appear in the terminal window, and the byte counters of the connection in TCP COM Bridge should grow. If nothing arrives, check the firewall on the server, the port number, and the serial parameters from step 2.

Options that matter for a permanent link

Start without a user. The Professional edition can run as a Windows service and opens the connections before anyone logs on. The Standard edition can start together with Windows after a user logs on.
Broken links. "TCP keep alive mode" (Professional edition) sends small check packets and closes a dead connection quickly so that the client can reconnect.
No data loss while the link is down. "Buffer data if TCP/IP port closed" keeps the data that arrives from the COM port and sends it first when the connection is back.
Many small packets. "Sending timeout" collects the data that arrives within the given number of milliseconds into one network packet. This helps when a device sends many short pieces.
A client that is not TCP COM Bridge. The options on the "Advanced" tab (transmit RTS and DTR state, transmit communication parameters, encryption, additional keep-alive packets, optimization for small data packets) work only when TCP COM Bridge runs on both ends, and the settings must be identical on both sides. Leave them off when the client is another program or a device.
A port that other programs also need. "Open on demand" opens the real COM port only while a network client is connected, and "Close after inactivity" releases it after the given number of milliseconds without traffic.

Serial to UDP

Select "UDP" as the protocol type to send the serial data as UDP datagrams. UDP has no connection and no delivery guarantee, so the receiving application must cope with a lost packet. It fits small packets that go to several receivers. Two details from the version history are worth knowing: a UDP server sends data only to clients that were active during the last 120 seconds, so a client that only listens must send something from time to time; and the program can process incoming multicast packets. UDP requires the Professional edition.

Limits

The program sends the raw data stream. It does not implement RFC 2217, so a network client cannot change the baud rate of the COM port; set it in step 2.
It does not convert protocols. Modbus RTU stays Modbus RTU inside the TCP connection. For Modbus TCP clients use MODBUS Gateway.
Several clients may connect to one server, but the device sees one stream. If two clients send commands at the same time, the device receives them mixed. See sharing a COM port between computers.
The network adds delay to every request and reply. Check the round trip with ping before you rely on a link for a protocol with short timeouts.
TCP COM Bridge runs on Windows. The other end can be any system.

Which edition do I need?

One or two serial devices over TCP work with the Standard edition, which allows two connections. More connections, the Windows service mode, UDP, the keep-alive packets and traffic encryption require the Professional edition. One computer can serve up to 255 COM ports: see the serial device server setup. Compare editions.

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