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CHAPTER 2: Introduction
2.3.2 I
NTERFACE
The Fiber Optic Modem has an RS-530 interface.
2.3.3 T
IMING
The modulation timing circuit supplies the transmit clock timing signal to the
encoder. Three clock sources are available:
• Internal Clock (INT)—from the modem’s internal crystal oscillator.
• External Clock (EXT)—from the DTE.
• Receive Loopback Clock (RCV)—recovered from the receive signal, looped
back as a transmit clock.
The Fiber Optic Modem uses a phase locked loop (PLL) circuit to recover jitter-
free data and clock sync from the optical signal.
2.3.4 A
SYNC
-
TO
-S
YNC
C
ONVERTER
The Fiber Optic Modem has an internal Async-to-Sync converter. Asynchronous
transmission is handled by the internal conversion of the data signal from
asynchronous to synchronous (in compliance with ITU V.22 bis). For this standard,
the modem compensates for any frequency deviation between the modem and the
DTE by adjusting the length of the stop bit of the async character.
The operation mode of the Async-to-Sync converter is chosen by setting the
proper character length and frequency deviation (via the DIP-switch bank).
2.3.5 D
IAGNOSTICS
Built-in diagnostics (complying with the V.54 standard) enable digital loopback,
remote loopback, and local analog loopback. You can activate diagnostics either
from the front-panel pushbuttons or via the DTE interface. The pushbuttons and
the DTE interface are enabled (or disabled) by the SW EN, REM, and ANA straps
on the printed board.
Test Pattern Generator and Receiver
The pattern generator/receiver provides for the testing of the local modem and
the communication link. When the PATT button on the front panel is pressed, the
pattern generator circuit sends out a standard 511-bit pseudo-random pattern. If
any errors are encountered, the ERR LED lights up.
