3.0 Functional Description (Continued)
The Manchester encoder accepts NRZ data from the The decoder detects the end of a frame when no more
controller or repeater, encodes the data to Manchester, mid-bit transitions are detected. Within one and a half bit
and transmits it differentially to the transceiver, through the times after the last bit, carrier sense is de-asserted.
differential transmit driver.
Receive clock stays active for five more bit times after CRS
goes low, to guarantee the receive timings of the controller
or repeater.
The Manchester decoder receives Manchester data from
the transceiver, converts it to NRZ data and recovers clock
pulses for synchronous data transfer to the controller or 3.7.14 REQ and RTX
repeater.
These pins allow for the direct control of both the pre-
The collision monitor indicates to the controller the
presence of a valid 10 MHz collision signal.
emphasis (REQ) and the transmit amplitude (RTX) of the
10BASE-T transmit signal. These pins should normally be
left floating, however, in applications where lower transmit
amplitudes are required, these pins should be pulled-down
to ground resistively. Conversely, for applications that
require higher transmit amplitudes, these pins should be
pulled-up to Vcc resistively. Figure 16 provides a simplified
functional diagram.
3.7.13.1 Manchester Encoder and differential driver
The encoder begins operation when the Transmit Enable
input (TX_EN) goes high and converts the NRZ data to
pre-emphasized Manchester data for the transceiver. For
the duration of TXE remaining high, the Transmit Data
(TXD) is encoded for the transmit-driver pair (TXU+/- or
TXS+/-). TXD must be valid on the rising edge of Transmit
Clock (TXC). Transmission ends when TX_EN deasserts.
The last transition is always positive; it occurs at the center
of the bit cell if the last bit is a one, or at the end of the bit
cell if the last bit is a zero.
Some experimentation is required in order to fully evaluate
the extent of transmit amplitude (and corresponding pre-
emphasis) variation due to the system to system variations
in external components (e.g. transformers and termination
networks). It is important to use the same resistor value for
both RTX and REQ (pulled to the same rail) in order to
allow the pre-emphasis to track the transmit amplitude.
3.7.13.2 Manchester Decoder
The decoder consists of a differential receiver and a PLL to
separate a Manchester encoded data stream into internal
clock signals and data. The differential input must be
externally terminated with either a differential 100Ω or
differential 150Ω termination network to accommodate
either UTP or STP cable respectively. Refer to Figure 16
for further detail.
In general terms, a value of approximately 50KΩ for these
pins (either pulled-up or pulled-down) will result in a
transmit amplitude (and pre-emphasis) change on the
order of 5% to 10%. Again, experimentation is
recommended.
Fixed
Vcc
Current
Source
R
R
RTX
Transmit
Amplitude
Control
Fixed
Current
Source
REQ
Transmit
Pre-emphasis
Control
R
R
GND
FIGURE 16. REQ and RTX Operation
Version A
National Semiconductor
32
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