Being a multi-function fiber modem in support of asynchronism RS-232, RS-485 and RS-422 communication interface, UT-277 is the best choice for the connection from remote terminal unit (RTU) to host (HOST) or Supervisory Control and Data Acquisition (SCADA). It supports multiple asynchronism communication protocols including RS-232, RS-485 and RS-422, and it also supports a combination of two of the RS-232, RS-485 or RS-422 interfaces as well as both of the working modes, i.e. Two-line (half-duplex) RS-485 and four-line (full duplex) RS-422. The ability to support RS-485 mode for data TXD or SD sending control greatly improves its compatibility with various kinds of software besides a sharp simplification of control method. Various kinds of fiber connection between asynchronism serial interfaces are supported by UT-277 fiber modem. Both half duplex and full duplex communications through fiber between two asynchronism interface devices are supported with a communication distance as far as 4 kilometers for multi-mode and 20 kilometers for single-mode. The transmission rate for RS-232 can reach a maximum of 115.2KBPS, while for RS-485/RS-422 a maximum of 460KBPS can be achieved. Interfaces of different electric standards can be mixed for use with nice EMI/RFT performance, e.g. RS-232 devices can be connected to RS-485/RS-422 devices, and the interface converter or photoelectric isolator can be replaced from RS-232 to RS-485. Two data signal transmissions are supported by UT-277: data sending and data receiving. At the same time, automatic control circuit is also provided for RS-485/RS-422 data transmission. Zero delay for converting time is achieved. ST interfaces are used for fiber connection.