
LXT9785 and LXT9785E Advanced 8-Port 10/100 Mbps PHY Transceivers
Datasheet 
Document Number: 249241
Revision Number: 007
Revision Date: August 28, 2003
145
4.9
100 Mbps Operation 
4.9.1
100BASE-X Network Operations
During 100BASE-X operation, the LXT9785/LXT9785E transmits and receives 5-bit symbols 
across the network link. 
Figure 27
 shows the structure of a standard frame packet. When the MAC 
is not actively transmitting data, the LXT9785/LXT9785E sends out Idle symbols on the line.
In 100BASE-TX mode, the device scrambles the data and transmits it to the network using MLT-3 
line code. The MLT-3 signals received from the network are de-scrambled and decoded, and sent 
across the RMII to the MAC.
In 100BASE-FX mode, the LXT9785/LXT9785E transmits and receives NRZI signals across the 
LVPECL interface. An external 100BASE-FX transceiver module is required to complete the fiber 
connection. 
As shown in 
Figure 27
, the MAC starts each transmission with a preamble pattern. As soon as the 
LXT9785/LXT9785E detects the start of preamble, it transmits a J/K Start-of-Stream Delimiter 
(SSD) symbol to the network. It then encodes and transmits the rest of the packet, including the 
balance of the preamble, the Start-of-Frame Delimiter (SFD), packet data, and CRC. Once the 
packet ends, the LXT9785/LXT9785E transmits the T/R End-of-Stream Delimiter (ESD) symbol 
and then returns to transmitting Idle symbols.
4.9.2
100BASE-X Protocol Sublayer Operations
In a 7-layer communications model, the LXT9785/LXT9785E is a Physical Layer 1 (PHY) device. 
The LXT9785/LXT9785E implements the Physical Coding Sublayer (PCS), Physical Medium 
Attachment (PMA), and Physical Medium Dependent (PMD) sublayers of the reference model 
defined by the IEEE 802.3u specification. The following paragraphs discuss the LXT9785/
LXT9785E operation from the reference model point of view.
4.9.2.1
PCS Sublayer
The Physical Coding Sublayer (PCS) provides the RMII interface, as well as the 4B/5B encoding/
decoding function. For 100BASE-TX and 100BASE-FX operation, the PCS layer provides IDLE 
symbols to the PMD-layer line driver as long as TxEN is de-asserted. For 10T operation, the PCS 
layer merely provides a bus interface and serialization/de-serialization function. 10T operation 
does not use the 4B/5B encoder. 
Figure 27. Intel
 LXT9785/LXT9785E 100BASE-X Frame Format
P0
P1
P6
SFD
64-Bit Preamble
(8 Octets)
Start-of-Frame
Delimiter (SFD)
DA
DA
SA
SA
Destination and Source
Address (6 Octets each)
L1
L2
Packet Length
(2 Octets)
D0
D1
Dn
Data Field
(Pad to minimum packet size)
Frame Check Field
(4 Octets)
CRC
I0
InterFrame Gap / Idle Code
(> 12 Octets)
Replaced by
/T/R/ code-groups
End-of-Stream Delimiter (ESD)
IFG
Replaced by
/J/K/ code-groups
Start-of-Stream
Delimiter (SSD)