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I am trying to establish a unidirectional connection over a RJ45 cable (not an optical fiber). I saw in the IEEE 802.3 standard that the clause 66 describe how unidirectionnal chips should work. However, I do not find any chip that implement this clause. So here are my 2 questions : - Do you know if there are some chips that are capable of establishing a unidirectionnal connection over a RJ45 cable? - Is it possible to reprogram an existing chip? Thank you!

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  • \$\begingroup\$ What are you trying to achieve using this connection? \$\endgroup\$ – Finbarr Apr 18 '17 at 13:36
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    \$\begingroup\$ Perhaps you don't find such chips because they would be useless for TCP which is 99% of IEEE 802.3 applications. \$\endgroup\$ – Dmitry Grigoryev Apr 18 '17 at 13:40
  • \$\begingroup\$ I just want to send data between 2 computers while one paired wire has been cut. \$\endgroup\$ – pierre123 Apr 18 '17 at 13:42
  • \$\begingroup\$ The use case would be to establish a secure network where you can only send data but not receive. \$\endgroup\$ – pierre123 Apr 18 '17 at 13:43
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    \$\begingroup\$ The problem is that most people sending data want to know that (a) the receiver is ready for it and (b) it's been received correctly. That requires a return connection for some sort of acknowledgement. Though I suppose you could use a UART at each end of the cable and just hope for the best. \$\endgroup\$ – Finbarr Apr 18 '17 at 13:47
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Do you know if there are some chips that are capable of establishing a unidirectionnal connection over a RJ45 cable?

At least any IEEE 802.3-compatible 100BASE-TX-enabled PHY could be programmed via the MII Management Interface (MDIO/MDC) to support one-way connection. The target configuration is as the following:

  • Disable Energy Efficient Ethernet (mandatory if supported)
  • Disable Auto-Negotiation (mandatory)
  • Force Full Duplex (mandatory)
  • Force 100 Mbps Mode (mandatory)
  • Force "permanent" Link Up (implementation-specific)
  • Disable Auto-Crossover (Auto-MDI/MDIX, implementation-specific)
  • Disable Far-End-Fault Generation (implementation-specific)
  • Disable (any) Power Save mode(s)/feature(s) (implementation-specific)
  • Disable (any) Cable Diagnostic feature(s) (implementation-specific)
  • Disable (all) other smart feature(s) (implementation-specific)

Here "mandatory" means that the feature is defined by the Standard, "implementation-specific" means that the feature may be, if presents, specific to the particular choice.

One additional trick for the transmitting side PHY if all the above (suddenly) has no enough effect: connect the RX± pins of the PHY to the TX± pins of the PHY - before or after the transformer is case-dependent. If the PHY has no internal termination you have a chance to terminate such a "local loop" only once. There is no need in such the trick for the receiving side PHY of a planned uni-directional connection.

Is it possible to reprogram an existing chip?

Yes, via MDIO/MDC if the chip you use is IEEE 802.3-compatible. The most (if not all) chips are.

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