SENA Serial_IPTM Redirector 4.3 - Manual (Page 30)

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TD is "lit" when transmitting data to the serial server.
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RD is "lit" when receiving data from the serial server.
Three more indicators appear only if the serial server supports COM Port Control:
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TR (DTR) is the signal to the serial port that the PC application has opened the Serial/IP COM port. The
most frequent use of DTR is to signal the serial server to disconnect by lowering the DTR line.
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DR (DSR) is the signal to the PC application that a serial device is connected to the serial server and
ready to communicate.
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CD (DCD) is the signal to the PC application from a device connected to the serial server that it has
successfully negotiated a connection with another device.
These indicators only appear when an application has opened a Serial/IP COM port. At other times, the indicator
area is blank.