20MBPS serial comms over 25 metres

[beetlejuice]

Hi All

I have a requirement to send a digital video over about 25m of cat 5 in an industrial environment. At the transmit end we have a frame buffer that captures a singe video frame 768 x 576 pixels 16bits wide. All in all just over 7MBits of data. I need to transfer this captured data over a cat 5 cable about 25m long. I need to transfer this amount of data about 2 to 3 times a second, which is where I arrive at the 20MBPS data rate.

I've looked at many SERDES chipsets but they just don't seem to cater for the lower frequency or the 25m transmission distance which is why i'm looking at alternative options.

I have 2 pairs of the cat 5 available to use for the data transfer. I have FPGAs at the transmit and receive ends (of course they will not be driving / receiving the signals directly ) and thought of implementing a high speed 20MBaud asynchronous link. I could use 2 individual channels to increase the bandwidth, (or reduce the Baud rate as necessary).

I could also try a synchronous transfer with one data channel and one synchronous clock, however I think the clock will have too much jitter.

I'm also unsure at this point about whats the best driver/receiver standard to use. My obvious first thought was RS422 level driver and receiver. I know that 20MBPS is pushing RS422 a bit and wonder if LVDS is a better option. (I have no experience with LVDS so am not sure how it will stand up in the industrial environment)


I'd appreciate any comments, or suggestions.

Thanks

 

[FvM]

I would either use UART protocol or synchronous transmission with embedded clock like Manchester or 8b/10b encoding. The latter can be also transformer isolated.

 

[beetlejuice]

Thanks for the advice FvM.

I've never really looked at Manchester Encoding but this does seem to be the best solution and should be quite easy to implement an encoder and decoder in the FPGAs.

Can anyone provide some advice on the use of RS422 or LVDS differential levels for this transmission over 25m?

 

[FvM]

Both can be used. I presume you reviewed datasheets about available RS-422 speed.

Personally I won't use either of it over 25 m without optical or transformer isolation, due to limited common mode capability of the receivers.

 

[beetlejuice]

Yes I have looked at the data sheets on various high speed RS422 devices and it does seem that 20MHz is pushing the limit.
I see that LVDS has a much higher operating frequency, but I have no experience with it. How suitable is it to drive a Cat5 pair over 25m?

In either case the signals will be isolated at the receiving end either using using high speed digital isolators, or as you have suggested by using Manchester coding transformers could be used. I have not been able to find suitably high frequency opto-isolators.

 

[FvM]

For data rates above 20 MBPS, you'll preferably use optical, capacitive or magnetical isolators that are available from many manufacturers, e.g. TI, Analog Devices, NVE, Avagotech. But transformers (ethernet magnetics) are the best option if you can generate DC balanced data.

LVDS links are impedance matched, 100 ohm differential. In so far the driver doesn't care if the CAT5 cable is 1 or 25 m length. There will be a certain signal distortion over cable distance, but it's sufficiently low at 20 (or manchester modulated 40) MBPS.

 

[beetlejuice]

Thanks for your help FvM.

It is greatly appreciated.