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SERDEC Question

Richard Bair
Prodigy 80 points
Other Parts Discussed in Thread: DS15EA101, DS15BA101, DS90CP02, DS25CP102

I have a customer who wants to connect mulitple signage tiles using 2' lengths of CAT6 cable using LVDS deserializers on each tile and then driving the LVDS signal to the next tile using a cable driver.  The customer has the following questions:

I'd like some advice on how to connect each LVDS receiver to LVDS Deserializer and LVDS cable driver on each tile.
If I use DS15EA101 as receiver, its output feeds into DS15BA101 and into SN65LVDS1224.
Since it is a 10bit system, documentation suggest for a DC coupled connection.
From DS15EA101 I run 2 wires straight into 1224 and I add a 100 ohm parallel resistor for line impedance.
Can these 2 wires go straight into DS15BA101 to reamplify the line ? Do they need AC coupled ?
What are the issues related to having 2 inputs (1224 and BA101) behind one output (EA101) ? Any schematic suggestion from TI ?
LVDS speed is in the 400Mbps range.

  • 0
    TI__Guru* 88835 points
    Hi Richard,

    DS15EA101 and DS15BA101 are point to point devices. Therefor we cannot connect the output of DS15EA101 to both SN65LVDS1224 and DS15BA101. To do this, perhaps you can connect the output of DS15EA101 to DS90CP02(2X2 1.5Gbps cross point). One output of DS90CP02 goes to SN65LVDS1224 while the other output goes to DS15BA101 input.

    Normally LVDS receiver has wide dynamic range so you can DC couple between DS15EA101 3.3V CML and 3.3V LVDS receiver. I was not able to find SN65LVDS1224 input common mode voltage range so please check to make sure it supports 3.3V dynamic range as shown in figure 1 or 2 of the DS15EA101 data sheet.

    Regards,,nasser
  • 0 in reply to Nasser Mohammadi
    Prodigy 80 points
    Thanks for the information Nasser. One more question:

    The normal length of the differential cable between 2 video tiles is in the range of 50cm. If I have a cross switch on each tile, is it still a good advice to add the BA101 and EA101 pair too accommodate for equalization or is it overkill because the switch would handle the cable and FR4 routing limitations ?
    I might want to be on the safe side and have a sound system but I'd like to hear your opinion in this matter.
  • 0 in reply to Richard Bair
    TI__Guru* 88835 points
    Hi Richard,

    What matters is the loss of your 50" media vs frequency. Given 400Mbps and 50cm cable length, you maybe able to do as you noted(without using DS15EA/BA devices). If this is the case, please consider DS25CP102. This device has output pre-emphasis and input equalization which can come in handy and you can tune during your development.

    Also, please note figures 8&9 and test channel characteristics. Please make sure your 50 inches media insertion loss is the same or better than the test channel loss vs frequency shown in the table.

    Regards,,nasser
  • 0 in reply to Nasser Mohammadi
    Prodigy 80 points

    The question is about how much pre-emphasis and equalization each set of devices can offer.
    My experience is zero in this field, please pardon me if it sounds stupid.
    DS15EA/BA101 devices seem to handle a wide range of length of cable while DS25CP102 has one setting. Will this be detrimental to our application ?
    I want to clarify how we are going to use LVDS in our application: LVDS will run through CAT5e or CAT6 cable, not sure about connectors yet but we like Molex Minifits because of handling but any suggestion from TI would be welcome, just keep in mind that it is industrial, outdoor, bad conditions and difficult access and small connectors don't last long because of rough people handling them. Budget and tooling are also very important factors.
    Most cables will be less than 75cm long between tiles but we could have a few header cables from the main controller to a first tile in a chain that could reach 5 meters at the most.
    Probably overkill to use DS15EA/BA101 but I like a good solution. Would DS25CP102 still be fine ? Would this one be fine from tile to tile and should we have a DS15EA/BA101 "extender" solution if we run into difficulties ?

  • 0 in reply to Richard Bair
    TI__Guru* 88835 points
    Hi Richard,

    After some search, i was able to find out 100m CAT5e has about 35dB of loss at 200Mhz. Given your maximum cable length is 5m and assuming loss is linear, this means your 5m CAT5e could have 3.5/2=1.75dB or a round number of 2dB loss maximum(@ 200MHz). I believe given this, DS25CP102 should be able to handle this. Please note device has not been characterized using this cable and length but given the loss behavior we should be able to handle this.

    Regards,,nasser
  • 0 in reply to Richard Bair
    TI__Mastermind 43795 points

    Hi Richard,

    At 100 MHz CAT5e cable will have a maximum attenuation of 24 dB/100meters or 0.24 dB per meter.

    At 200 MHz this increases to roughly 32 dB/100meters or 0.32 dB per meter.

    Based on this infomation, your 400 Mbps data should be fine with respect to attenuation or loss in the cable.  I do not think you need additional buffering, differential signals should be able to tolerate ~ 3 dB of loss without significant degradation of performance.  I would be more concerned with the connectors - the Molex Minifit seem to be power connectors which are not really specified for high speed data.  This could cause crosstalk and other negative signaling effects on the video signals.

    I would investigate industrial / automotive connectors which are designed for data transfer. 

    Regards,

    Lee