I would like to use an Ethernet jack for the form factor and CAT5e+ cable properties (shielded/twisted, characteristic impedance, etc) to send LVDS signals over a long length of cable (100+ ft). When choosing board Ethernet connectors, I see that there are all sorts of types out there. I am going to use the RJ45 T-568B wiring scheme, but then there are choices between magnetics, and I assume, no magnetics. Can I take advantage of the magnetically isolated jacks with an LVDS signal (coming from SN65LVDS31 driver), or will it likely interfere? My speeds will be in the 25 Mbps range. If the magnetic jacks will interfere, are there standard pin to wire type RJ45 connectors (no magnetics)?
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\$\begingroup\$ I believe it will work. LVDS cannot be transformer coupled, so no magnetics. You can use a common mode choke on each pair if you want. Termination will be very important. You need to try this before you assume it will work. \$\endgroup\$– user57037Feb 1, 2017 at 17:38
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4\$\begingroup\$ Also, technical issues aside, you're going to have to be very careful that no one ever plugs an actual Ethernet cable into your jack, especially one that has power (PoE) on it! Why not pick a connector explicitly designed for LVDS? e.g., DVI, HDMI, eSATA, etc. \$\endgroup\$– Dave TweedFeb 1, 2017 at 18:15
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1\$\begingroup\$ @mkeith, I was under the assumption that transformer coupling is only an issue when the data isn't DC balanced, as long as the common mode range is adhered to and a DC balanced protocol is used (like 8b/10b) it should be OK, otherwise we wouldn't see AC coupled LVDS systems like SATA and PCIe. \$\endgroup\$– SamFeb 1, 2017 at 19:12
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1\$\begingroup\$ @mkeith By termination, you are referring to termination resistors (e.g. 100 ohm across the receiving end), correct? Is there any special consideration I need to think about? Again, just interested in the Ethernet cabling, not the actual properties of Ethernet or serial communication. \$\endgroup\$– jareddbhFeb 1, 2017 at 21:34
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1\$\begingroup\$ Yes. 100 Ohms at the receiving end. It has been a while since I used LVDS. But it may be desirable to use source termination also, so that you end up with a "doubly terminated" line. As long as the driver is capable of driving it. \$\endgroup\$– user57037Feb 1, 2017 at 21:41
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1 Answer
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LVDS signals are generally incompatible with transformers/magnetics. The Texas Instruments application note does not have any examples of this sort.
Here is a similar discussion, see this Xilinx thread.
The LVDS will likely work over 100ft of RJ45/CATwhatever connection, you just need no magnetics and correct termination scheme on driver and receiver sides. However, hot plug/reconnect could be problematic due to possibility of cable charge up and ESD killing your driver/receivers.
answered Feb 1, 2017 at 17:53
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1\$\begingroup\$ You've stated that "LVDS signals are generally incompatible with transformers/magnetics". Could you elaborate on that? Especially as the Xilinx thread you linked to contains the advice "yes you can use transformers [with LVDS]. And it works, I've got the T shirt" right in the second post. Yes, it's tricky to properly impedance match a transformer, you can get big spikes, you must use a DC balanced protocol, but as long as good design practices are adhered to (and the OP's running at 25Mbps so the matching isn't too bad - not like Gbps signalling) why would magnetics cause problems? \$\endgroup\$– SamFeb 1, 2017 at 19:25
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\$\begingroup\$ @Sam, I believe you just answered your question yourself, by outlining several areas of specific concerns. And for 25MHz it could be even more difficult, saturation can occur, standard magnetic can be too small for this low frequency, etc, etc. \$\endgroup\$ Feb 1, 2017 at 19:57
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\$\begingroup\$ @AliChen Thank you for the response. I know very little about Ethernet, but I like the form factor (small easy to use plugs), and I like the well documented characteristics of CATx cable. From what I'm gathering from the conversations, I should be good with using this type of plug as long as it is just a straight feed-through with no magnetics. Does this sound right? \$\endgroup\$– jareddbhFeb 1, 2017 at 21:29
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1\$\begingroup\$ @AliChen, Would you recommend adding some TVS (transorb) diodes on the outputs of my LVDS to prevent hot-plug damage? In this case, would I just send each output line (4 differential pairs, 8 lines) to a TVS diode to ground? \$\endgroup\$– jareddbhFeb 14, 2017 at 16:44
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1\$\begingroup\$ @jareddbh, look at industrial examples how HDMI and DVI signals are protected, these are hot-plug ports. \$\endgroup\$ Feb 14, 2017 at 17:20