ML405 - TX_Disable input to SFP module can be driven to undefined state
Description
Keywords: ML405, SFP, TX_DISABLE, TX Disable, SFP_TX_DISAB, 1000BASE-X
A problem with the ML405 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 resistor, R320, on the ML405 board acts as 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 causes 1.65V to be applied to the TX_DISABLE pin which is an ?undefined? logic state.
Solution
To work around this issue, replace R320 with a 1K ohm 0603 resistor to remove the potential for an ?undefined? logic state on the SFP TX Disable pin.1. R320 is located on the bottom side of the board (near the back side of the SFP cage).
2. Below is an image of the Bottom Side of ML405 board. R320 is highlighted Red. Replace with 1K ohm 0603 resistor.
Top Left of the Bottom Side of the ML405 board