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AR# 30038

ML555 - TX_Disable input to SFP module is driven to undefined state


A problem with the ML555 SFP TX Disable circuit has been identified. The SFP/MSA specification states that all SFP modules have an internal 4.7K PullUp on the SFP TX Disable pin. The ML555 has a PullDown on the SFP TX Disable pin external to the SFP module. Since there is already a 4.7K PullUp inside the SFP and a 4.7K PullDown external to the SFP TX Disable pin, this can cause the TX_DISABLE pin to go to an "undefined" logic state.


As a work-around, replace R402 and R394 with 1K ohm resistor.

AR# 30038
Date Created 09/17/2008
Last Updated 12/15/2012
Status Active
Type General Article