On Fri, 27 Jul 2012, Alexey Filin wrote: > So USB is not a "classic" bus and not universal, it is a network with > non-constant delay. > > Are there USB controllers with the "synchronous transfers" reported > not on microframe boundary, so we can get response less 125 us? > Is there any USB2.0 spec extension declaring "synchronous transfers"? > > USB is a good choice for some bus adapters, it is used widely, cheap, > simple. For 2-byte read/write we could use either two bulk IN/OUT > endpoints (131 * 8kHz ~ 1 MHz) or one control endpoint (table 5-3, 42 > * 8 kHz = 336 kHz). Ethernet is overkill for the task even if it could > provide 1 us delay. Gigabit ethernet can transfer around 1000 bits every microsecond. In theory that's enough to handle two 2-byte transfers. But I don't know what the latency is; probably it depends on the hardware and the driver. > Is it possible for USB Implementers Forum to add extension for > "synchronous transfers" to be handled asynchronously with microframe > boundaries? > Existing bus adapters use frequently closed proprietary protocols for > pc-adapter link, e.g. CAEN CONET for PCI(e)-VME (I talk about remote > adapters to access with a link, not bridges)... Peter's response was right on the money. USB wasn't designed to do what you want and it wasn't intended to be used that way. You should look into a different technology. I don't know offhand what schemes offer latencies under 1 us, however. With a general-purpose OS like Linux, even with the realtime modifications, you'll find it very difficult to achieve such a low latency. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
