communicationethernetoptical-fibre
I want to build a unidirectional comminucation. In transportation layer of this device, it needs a one-way media to send data from side A to side B.
All commercial products use SFP. I tested some SFP devices but they have negotiation between the two sides and cutting RX fiber is not possible.
Is it possible to implement One-way connection with CAT5 cable?
SideA { tx –> rx } SideB
CAN sounds the most applicable in this case. The distances inside a house can be handled by CAN at 500 kbits/s, which sounds like plenty of bandwidth for your needs. The last node can be a off the shelf USB to CAN interface. That allows software in the computer to send CAN messages and see all the messages on the bus. The rest is software if you want to present this to the outside world as a TCP server or something.
CAN is the only communications means you mentioned that is actually a bus, except for rolling your own with I/O lines. All the others are point to point, including ethernet. Ethernet can be made to logically look like a bus with switches, but individual connections are still point to point and getting the logical bus topology will be expensive. The firmware overhead on each processor is also considerably more than CAN.
The nice part about CAN is that the lowest few protocol layers are handled in the hardware. For example, multiple nodes can try to transmit at the same time, but the hardware takes care of detecting and dealing with collisions. The hardware takes care of sending and receiving whole packets, including CRC checksum generation and validation.
Your reasons for avoiding PICs don't make any sense. There are many designs for programmers out there for building your own. One is my LProg, with the schematic available from the bottom of that page. However, building your own won't be cost effective unless you value your time at pennies/hour. It's also about more than just the programmer. You'll need something that aids with debugging. The Microchip PicKit 2 or 3 are very low cost programmers and debuggers. Although I have no personal experience with them, I hear of others using them routinely.
I see some recommendations for RS-485, but that is not a good idea compared to CAN. RS-485 is a electrical-only standard. It is a differential bus, so does allow for multiple nodes and has good noise immunity. However, CAN has all that too, plus a lot more. CAN is also usually implemented as a differential bus. Some argue that RS-485 is simple to interface to electrically. This is true, but so is CAN. Either way a single chip does it. In the case of CAN, the MCP2551 is a good example.
So CAN and RS-485 have pretty much the same advantages electrically. The big advantage of CAN is above that layer. With RS-485 there is nothing above that layer. You are on your own. It is possible to design a protocol that deals with bus arbitration, packet verification, timeouts, retries, etc, but to actually get this right is a lot more tricky than most people realize.
The CAN protocol defines packets, checksums, collision handling, retries, etc. Not only is it already there and thought out and tested, but the really big advantage is that it is implemented directly in silicon on many microcontrollers. The firmware interfaces to the CAN peripheral at the level of sending and receiving packets. For sending, the hardware does the colllision detection, backoff, retry, and CRC checksum generation. For receiving, it does the packet detection, clock skew adjusting, and CRC checksum validation. Yes the CAN peripheral will take more firmware to drive than a UART such as is often used with RS-485, but it takes a lot less code overall since the silicon handles so much of the low level protocol details.
In short, RS-485 is from a bygone era and makes little sense for new systems today. The main issue seems to be people who used RS-485 in the past clinging to it and thinking CAN is "complicated" somehow. The low levels of CAN are complicated, but so is any competent RS-485 implementation. Note that several well known protocols based on RS-485 have been replaced by newer versions based on CAN. NMEA2000 is one example of such a newer CAN-based standard. There is another automotive standard J-J1708 (based on RS-485) that is pretty much obsolete now with the CAN-based OBD-II and J-1939.
Most devices in the last decade do not have plain RJ45 jacks. Due to micronization of components and the demand for smarter/smaller parts, RJ45 jacks have combined the Jack with the Magnetic Couplers that used to be separate parts.
Typical schematic for a Mag Jack:
That is a 2 pair, 100Base-T version. Some have the other 2 pairs connected to pins instead of shield. And Gigabit or 4 pair versions simply have 2 more sets of magnetic couplers.
Notice the IC next to this jack. That is an external magnetic coupler/transformer. If the router or device you pull your jack from does not have something similar, it most likely has a MagJack.:
Best Answer
One-way Ethernet cables won't work with Gigabit network equipment and later, because without a return path the autonegotiation sequence will never complete. You'll see a "Network cable unplugged" or an equivalent message on both devices if you try to use such a cable.
Older Ethernet devices won't work with simple one-way cables either, but can be fooled to do so. This website has practical instructions for building suitable cables. In the essence, you'll have to do something like this:
The extra Hub port is needed to provide voltage over the RX pair of SideA, to fool it into believing there's a transmitter on the other end.
If none of the above works, or you cannot disable auto-negotiation, the next best thing is to implement two Ethernet-enabled devices with a one-way data path between them. For example, a couple of Ethernet PICs with a unidirectional UART in between should be able to do the trick.
One thing you should ask yourself before you proceed is whether you really need a data diode. Here's what the experts say: