DS90LV011A: M-LVDS query

Hi Prahlad,

Using some form of LVDS or MLVDS to supply a 200 Mbps communication between racks is entirely possible if the link is Point-to-Point (one Tx to one Rx).  If this link is setup as any part of a multi-drop backplane interconnect, then running at this speed will be virtually impossible using this technology.  The device loading and backplane stubs will degrade the signals to much below the device maximum of 200 Mbps.

Just want to check on the cable length, 100 meters?

Regards,

Lee 

Hi Lee,

We are planning to use multi-drop LVDS to have 1 Master which can be able to communicate with multiple Slaves. Attached is the scheme which we are planning to use. 

There are two queries related to this architecture:

1. If we are using this in the backplane, how much maximum speed we can achieve considering the length of the backplane to be around 460 mm?

2. If we want to extend to other rack using a cable of length 100 meters, what speed we can achieve?

Regards,

Vivek

Hi Vivek,

Placing the Master1 at one end of the backplane will help to retain bandwidth across the interface.  The termination would need to be at the far end at Slave12.

1. The overall density of the backplane is good for bandwidth.  Approximately 1 load every 3.5cm  (460mm / 13).   The biggest bandwidth constraint tends to be the stub length between each slave and the backplane itself.  These devices have IBIS models.  Using a tool like hyperlynx could give you good insight into the potential bandwidth of the backplane with this topology.

2. I would not recommend using this technology for an extension of 100 meters.  I would use something like Ethernet to make long distance connections.

Regards,

Lee

Hi Lee,

Thank you quick response. Few more points as mentioned below:

1. Yes, we will be using terminations, based upon the no. of load being used. Also, based upon the load type we will create IBIS model and simulate to find achievable bandwidth.

2. If at all required to use an extension cable, what maximum length we can go for without degrading bandwidth? Like 10m or 40m?

    Or we should not use multi-drop LVDS protocol when using a cable, only limit it to backplane?

Regards,

Vivek

Hi Vivek,

It is more than just the attenuation.  LVDS is really not designed for long distance communication.  The common mode voltage is too narrow of a voltage range. This leads to a fragile implementation.  To overcome GND differences, Ethernet uses isolation transformers for this type of isolation.

If the cable connections are within the same room or area so that the two equipment racks can be tied to the same earth GND then using MLVDS or LVDS across a cable is possible.  Otherwise, I would not recommend LVDS. 

Regards,

Lee