100mbps LVDS from XMOS?

[Rocketmagnet]

Hi,

I'm interesting in implementing a high speed bus to connect several PCBs together. I'd love to use EtherCAT but I also need to keep the parts cost and size down as much as possible. So I'm looking for a solution that doesn't require a separate processor and EtherCAT ASIC (e.g. ET1200). I've seen that there has been some work on making an EtherCAT component for the XMOS, but this is a PHY based one. EtherCAT also supports an LVDS physical layer.

My question:
Theoretically, is it plausible to implement something like EtherCAT over LVDS directly from an XMOS processor?

- Could it update/sample a pin at sufficient rate for 100mbps Manchester encoded data (250M updates per second)?
- Do XMOS chips have LVDS I/O, or would I nees an LVDS transceiver?

Many thanks in advance

Hugo

[Bianco]

- no
- no

If all PCB's have an XMOS processor you can connect them using one or more XMOS xCONNECT links (up to 400mbit/s for 5-wire/direction link)

[segher]

Also, 100Mbps is easy to do on just a few pins,
there is no need to use LVDS, it does not help
but does hurt!

[TSC]

What sort of distance would the boards be from each other? I don't think there are any XMOS applications where the xCONNECT links are longer than about 300 mm. Even on the Slicekit Core boards there are logic level buffers on the xCONNECT lines.

[Rocketmagnet]

Also, 100Mbps is easy to do on just a few pins,
there is no need to use LVDS, it does not help
but does hurt!

Thanks for the reply segher. Can you elaborate. You mean there are a few pins capable of doing this? Or there is enough processing power to do it on a few pins? In what way does LVDS hurt? As I understand LVDS is excellent for medium distance transmission, especially in regards to EMI.

[Rocketmagnet]

What sort of distance would the boards be from each other? I don't think there are any XMOS applications where the xCONNECT links are longer than about 300 mm. Even on the Slicekit Core boards there are logic level buffers on the xCONNECT lines.

Typical distances would be from 30mm to 2m.

[lilltroll]

The old XDK development kit (before slice-KIT) had LVDS equipped xConnect port. You could connect several kits with CAT6 cable.
I think you may find some schematics on that.
You may also take a look at this, Chapter 7:
https://www.xmos.com/en/download/public ... 99B%29.pdf

[segher]

Also, 100Mbps is easy to do on just a few pins,
there is no need to use LVDS, it does not help
but does hurt!

Thanks for the reply segher. Can you elaborate.

I'll try :-)

You mean there are a few pins capable of doing this?

All I/O pins are the same (modulo port assignments; and a few
of the 1-bit ports have higher drive).

Or there is enough processing power to do it on a few pins?

Yes, 100Mbps is peanuts.

In what way does LVDS hurt?

You need twice the number of pins, and special voltages.

As I understand LVDS is excellent for medium distance transmission, especially in regards to EMI.

Yes. I thought you wanted to connect PCBs, e.g. via an edge
connector; but you want a longish cable. LVDS is nice for
that indeed; you will need some transceiver chip.