
LXT9785 and LXT9785E Advanced 8-Port 10/100 Mbps PHY Transceivers
Datasheet 
Document Number: 249241
Revision Number: 007
Revision Date: August 28, 2003
131
4.6.1.5
Parallel Detection
In parallel with auto-negotiation, the LXT9785/LXT9785E also monitors for 10 Mbps Normal 
Link Pulses (NLP) or 100 Mbps Idle symbols. If either symbol is detected, the device 
automatically reverts to the corresponding operating speed in half-duplex mode. Parallel detection 
allows the LXT9785/LXT9785E to communicate with devices that do not support auto-
negotiation.
When parallel detection resolves a link, the link must be established in half-duplex mode. 
According to IEEE standards, the forced link partner cannot be configured to full-duplex. If the 
auto-negotiation link partner does not advertise half-duplex capability at the speed of the forced 
link partner, link is not established. The IEEE Standard prevents forced full-duplex-to-half-duplex 
link connections.
4.6.1.6
Reliable Link Establishment While Auto MDI/MDIX is Enabled in Forced 
Speed Mode
With auto MDI/MDIX hardware enabled, end users experience reliable link establishment under all 
settings of auto MDI/MDIX and speed between the LXT9785/LXT9785E and its link partners. As 
stated in the IEEE clauses 40.4.5.1 (Auto MDI/MDIX) and 28.3.2 (Parallel Detect), when ports are 
forced to 10 Mbps or 100 Mbps and auto MDI/MDIX is enabled, and the port is connected to a 
partner with auto-negotiation enabled, an undefined condition exists between the IEEE auto MDIX 
and Parallel Detect specifications. Link may not occur according to the IEEE specification.
During this undefined condition, when the LXT9785/LXT9785E is set to 10 Mbps or 100 Mbps 
and auto MDI/MDIX is enabled, the LXT9785/LXT9785E and the link partner auto-negotiation 
processes are expected to be skewed enough to establish link in all but the rarest cases. Auto MDI/
MDIX is configured through hardware and software. If auto MDI/MDIX operation is desired in 
forced modes, disabling auto MDI/MDIX using the software programming can aid link 
establishment.
Figure 14. Intel
 LXT9785/LXT9785E Auto-Negotiation Operation
Check Value
0.12
Start
Done
Enable
Auto-Neg/Parallel Detection
Go To Forced
Settings
Attempt Auto-
Negotiation
Listen for 10T
Link Pulses
Listen for 100TX
Idle Symbols
Link Set
NO
YES
Power-Up, Reset,
Link Failure
Disable
Auto-Negotiation
0.12 = 0
0.12 = 1