THVD1450: RS485 / RS422 driver for 30' UL2919 cable

I meant transceivers. 

Hi Max;

Thank you very much.

Our sales application engineer: Jesse woolridge (who is copied in this correspondence) informed me that we are looking for "at least 40 Mbps over 30 feet of SCSI2 (UL2919) cable". I guess this leaves us with two options: THVD1450 &  SN65HVD78 (unless you can recommend other transceivers). Can we use these two transceivers in this application?

What are these drivers maximum output differential and single ended (against GND) capacitance?

Regards;

Karo

Max,

As Karo mentioned, our immediate need is to reach 40Mbps over 30-feet of standard SCSI2 type cable.  Ideally, we would want to reach 50Mbps at 30-ft, and perhaps 40Mbps up to 100-feet.  Please note:

• If there is a better signal to use for data rates that high at those distances (perhaps LVDS?), we can consider that as well.
  • Ideally if there is an LVDS component that is drop-in footprint compatible with a '485 version, that would allow us to use the same base design for multiple applications.
• We should be able to use controllable skew/emphasis/EQ if that is recommended
• Because of the particulars of our requirement, the cable type is common SCSI2.  That is firm.

We appreciate any feedback you can provide.  Thank you!

-Jesse

Hi Jesse,

Thanks for the additional info.  These lengths seem generally within a range that can be achieved with RS-485 at these data rates, although if you wanted to also evaluate a higher-speed protocol you might look into something like M-LVDS.  (It's similar to LVDS but developed for multipoint applications and features a higher signaling swing that could be useful in extending the reach in this case.)  Luckily some of those devices are footprint-compatible to RS-485 - for example, THVD1450 (the RS-485 device I'd recommend here) and SN65MLVD204B (the M-LVDS device I'd recommend) have compatible packages.

Unfortunately unless you have an accurate picture of the insertion loss of the cable it may require some lab analysis to really confirm that either option would work well.  One resource I found mentioned that an example SCSI-2 cable had a loss of 2.8 dB/10 m at 50 MHz.  If we make the rough assumption that the loss is about half this at 25 MHz (the Nyquist frequency of your 50 Mbps data rate), then you'd have <2 dB of loss for your 30-ft use case.  That would certainly be manageable.  For 100 ft (~30 m), though, you could have >30 dB at the Nyquist rate.  This would likely result in too much jitter to be manageable.  Of course, there's only one way to find out for sure and that's lab testing.

Note that there are some simple ways to implement equalization external to a transceiver - see this reference design for an example:

http://www.ti.com/tool/TIDA-00790

There are also some devices with this built in like SN65HVD23, although that device in particular is rated for a maximum transmission rate of 25 Mbps (and there unfortunately aren't any "faster" options in the RS-485 portfolio with internal equalization).

Regards,
Max