Specifications

Chapter 1 Introduction FOM-40 Installation and Operation Manual
1-4 Functional Description
1.3 Functional Description
This section contains functional descriptions of the FOM-40 circuit blocks shown
in Figure 1-4.
Figure 1-4. FOM-40 Block Diagram
FOM-40 provides a high-speed data link between computers, routers, or
multiplexers. The electrical signals from the DTE unit are converted into an optical
signal using an infrared light emitting diode. At the opposite end of the fiber, the
optical signal is converted back into an electrical signal in compliance with the
appropriate interface.
Timing
The modulation timing circuit supplies the transmit clock timing signal to the
encoder. Three timing sources are available:
• Internal, supplied by the internal crystal oscillator
• External, derived from the DTE clock
• Receive, recovered from the received signal.
FOM-40 utilizes a Phase Locked Loop (PLL) circuit to recover jitter-free data and
clock from the optical signal.
Diagnostics
FOM-40 provides remote digital loopback, local analog loopback, and local digital
loopback. The loopbacks are activated manually from the front panel. Additionally,
remote digital loopback and local analog loopback are activated over the DTE
interface using pins ”h” and “j” of the V.35 or pins 18 and 21 of the RS-530 or
RS-232. These pins are enabled or disabled separately by the DTE DIP switch (LLB
and RLB).
Test Pattern Generator and Receiver
The Test Pattern Generator easily and quickly tests the local modem and the
communication link. When the PATT pushbutton on the front panel is pressed, the
circuit sends a standard 511-bit pseudo-random pattern and checks the response. If
errors are detected, the front panel ERR LED indicator remains ON or blinks.