Solution:

The ESP901, when used TCP Socket mode, and in RS-232 mode will hold the RTS line in the negative voltage state when the TCP Socket connection is closed. ( roughly -6 volts ).When the TCP Socket connection is opened, the RTS line will go to the Positive state, ( roughly +6 V).Then when the socket connection is closed, it will go back to a - 6 V level.( When Flow control is set to none)NOTE: The ESP211 series does the same. NOTE: The VESP and VESR devices hold the RTS line in the + 6 V level, and do not change when the TCP Socket is opened or closed.