RTL8208B-LF/RTL8208BF-LF
Datasheet
7.4.3. Link Monitor
In 100Base-TX mode, receive signal energy is detected by monitoring the receive pair for transitions in
the signal level. Signal levels are qualified using squelch detect circuits. When no signal or valid signals
are detected on the receive pair, the link monitor will enter and remain in the ‘Link Fail’ state where only
idle codes will be transmitted. When a valid signal is detected on the receive pair, the link monitor will
enter the ‘Link Pass’ state and the transmit and receive functions will be enabled.
7.5. Baseline Wander Compensation
The RTL8208B(F)-LF is ANSI TP-PMD compliant and supports input and Base Line Wander (BLW)
compensation in 100Base-TX mode. The RTL8208B(F)-LF does not require external attenuation circuitry
at its receive inputs, RXIP/RXIN. It accepts TP-PMD compliant waveforms directly, requiring only a
100Ω termination and a 1:1 transformer.
BLW is the change in the average DC content, over time, of an AC coupled digital transmission over a
given transmission medium. BLW is a result of the interaction between the low frequency components of
a transmitted bit stream and the frequency response of the AC coupling component(s) within the
transmission system. If the low frequency content of the digital bit stream goes below the low frequency
pole of the AC coupling transformers, then the droop characteristics of the transformers will dominate,
resulting in potentially serious BLW. If BLW is not compensated for, packet loss will occur.
7.6. 100Base-FX (RTL8208BF-LF Only)
The RTL8208BF-LF (the RTL8208B-LF does not support fiber application) can be configured to
100Base-FX mode through SEL_TXFX[1:0] (PP-LPBK MODE should be 0), depending on the setting of
SEL_TXFX[1:0], port 7 or port 6/7, or all eight ports can be configured for 100Base-FX operation.
Note: In compliance with IEEE 802.3u, 100Base-FX does not support Auto-Negotiation. In order to
operate correctly, both sides of the connection should be set to the same duplex and flow control ability.
Table 24. 100Base-FX
PP-LPBK Mode=0
SEL_TXFX[1:0]
Medium Type
Port 0 Port 1
Port2
UTP
UTP
UTP
FX
Port3
Port4
UTP
UTP
UTP
FX
Port5
UTP
UTP
UTP
FX
Port6
UTP
UTP
FX
Port7
UTP
FX
FX
FX
2’b00
2’b01
2’b10
2’b11
UTP
UTP
UTP
FX
UTP
UTP
UTP
FX
UTP
UTP
UTP
FX
FX
UTP: 10Base-T/100Base-TX. FX: 100Base-FX.
The RTL8208BF-LF performs a link monitoring function with less pins required than regular
100Base-FX applications (it removes a pair of differential Signal Detect signals (Realtek patent)), which
significantly reduces the pin count in this octal PHY.
Any of the RTL8208BF-LF transceivers may interface with an external 100Base-FX fiber optic device
and receiver, instead of the magnetic module used with twisted-pair cable. The differential transmit and
receive data pairs will operate at PECL (Positive Emitter Coupled Logic) voltage levels instead of those
required for twisted-pair transmission. The data will be encoded using two-level NRZI instead of three-
level MLT3. The data stream is not scrambled for fiber-optic transmission.
Single-Chip Octal 10/100-TX/FX PHY Transceiver
31
Track ID: JATR-1076-21 Rev. 1.3
REALTEK [ Realtek Semiconductor Corp. ]
