On Thu, Mar 31, 2016 at 5:29 PM, Slichter, Daniel H. (Fed) <daniel.slich...@nist.gov> wrote: >> We'll probably want a few dozen TTLs, broken out on SMA, so the FMC panel >> is not an option there. >> >> We can remove PCIe indeed, but keeping the WR oscillators is probably a >> good idea as they can be used for clock synchronization with the master. > > For the purpose of a TTL card, I would recommend that the TTL be broken out > to LVDS over cat5/cat6 using RJ45 connectors, as is currently done in the > ARTIQ hardware. It would be possible to send 64 TTL lines out of a single > AMC card of 6 HP width in this manner, much more than you could ever do with > SMA, and with vastly cheaper cabling and excellent signal integrity for long > cabling runs (tested to work fine with 30 m cable, for example). We have > existing breakout boards that convert between 4 TTL signals on SMA and 4 LVDS > signals on Ethernet cables. > > This card would not have an FMC mezzanine, but would rather just break things > out directly from the FPGA. I would recommend using a similar architecture > on the AMC board to our existing TTL riser card that interfaces between TTL > at the FPGA and LVDS. I know we could directly drive LVDS to/from the FPGA, > but then we don't have any isolation between the FPGA user IO and the end > user application, which makes me nervous that users could more easily fry the > FPGA. > > One could use a very inexpensive FPGA for this particular task, although it > might be nice to have a hard processor if it is driving so many TTL lines.
Since this is another piece of hardware and the processing constraints as well as the electrical constraints are so different, it seems prudent to account for these differences. Consider doing proper galvanic isolation with a fiber: ground potential differences easily -- and even in well controlled labs -- exceed the common mode tolerances of lvds if the devices are a few tens of meters apart. This is why we would like to consider a very low barrier, non-rack form factor that is connected by fiber plus a simple power supply and provides a good number of analog voltages and a good number of ttls. That obsoletes the LVDS breakout board which also doesn't help with the galvanic isolation for the high density low speed DAC that we would like to bundle with that box. Robert. _______________________________________________ ARTIQ mailing list https://ssl.serverraum.org/lists/listinfo/artiq
