How can I connect 1000BASE-SX (multi-mode fiber) to 100BASE-TX/10BASE-T (copper)?

Question

How can I connect an optical 1000BASE-SX SFP optical transceiver to a microcontroller (MCU) with 100/10BASE-T? When using a media converter like DP83869 both sides of the link must be configured to the same operating speed, i.e. 1000BASE:

"The DP83869HM supports the 1000Base-X Fiber Ethernet protocol as defined in IEEE 802.3 standard. In 1000M Fiber mode, the PHY will use two differential channels for communication. In fiber mode, the speed is not decided through auto-negotiation. Both sides of the link must be configured to the same operating speed."

I can not use another 100BASE-FX Tranceiver, it has to be 1000BASE-SX SFP. Will a switch after the DP83869 do the job of buffering 1000BASE-T into 100BASE-TX when the traffic is not high?

Answer 1

A switch with appropriate ports would do the job of converting the speeds for you. I'm not sure how much building it on the board is a requirement but if it isn't then some media converter like an AT-MMC2000/LC would do the job for you.

As for buffering, that's not really the switch's job to any significant extent. Not sending the microcontroller packets faster than it can process them is down to the thing that it is communicating with. Usually leaving it up to TCP works well enough, and indeed putting too much intermediate buffering in just makes it harder on the TCP rate control loop.

Answer 2

How can I connect an optical 1000BASE-SX SFP+ optical transceiver to a microcontroller (MCU) with 100/10BASE-T?

There are two criteria that need to match: media conversion (multi-mode fiber to copper), and speed adaptation via a switch.

You can use either a switch with a 1000BASE-SX SFP module and 10/100 or 10/100/1000 copper ports, or a switching media converter. Normal media converters cannot convert different speeds.