 remote unit, setting a transition timer for an RS-485 connection, and enabling primary and secondary optics. The RS-485 transition timer is set by three DIP switches to a predefined value based on the asynchronous data rate to be supported. This setting is required if you are using a two-wire RS-485 interface, as this represents a half-duplex operation in Fiber in the Building which the transmit and receive operations take turns. Concerning the optical connections supported by the TC1540, if you again refer to Figure 8.2, you will note two pairs of optic connectors at the lower portion of the figure. The pair of connectors on the right side are ST fiber-optic connectors and represent the primary optics that are enabled by one DIP switch. Thus, a pair of TC1540 fiber-optic modems requires two fibers, one for transmission from one modem to the other while the second fiber enables one modem to receive transmission from the other device. The second pair of connectors to the left of the primary connectors represents an optional optic redundancy feature that is included only when a dual-optic TC1540 fiber-optic modem is ordered. Here the second connector pair can be obtained as FC con- nectors and provides a mechanism for supporting critical applications. The TC1540 supports multimode fiber at wavelengths of 850 or 1310 nm as well as single-mode fiber at wavelengths of 1310 or 1550 nm. LED INDICATORS The LED indicators visually denote the presence of power to the unit, an alarm indication which results in a flashing signal when the optical signal is lost, as well as the transmission and reception of data on both the electrical interface and optical interface. Thus, the LEDs provide a visual indication of the operational state of the modem. In fact, if you obtain a pair of TC1540 fiber-optic modems with dual-optic connectors and one cable breaks, not only do the modems switch over to the secondary link but, in addition, the alarm LED will flash to indicate the cable break. TRANSMISSION DISTANCE While the primary use of