Hi Linux USB serial port experts,
I recently bought a 4-port USB RS485 serial port box. It is immediately
recognized by our Linux machines and works fine for the simple things. My
particular application uses RS485 on a monitor and control bus for telescope
control and has strict timing requirements. For this device to work it will
need to respond with an ACK signal within ~500 microseconds of receiving the
last byte of an address word. We are operating at 57600 baud.
Reading a knowledgebase article at
http://www.ftdichip.com/Support/Knowledgebase/index.html (transcript of page
below [*]) I learned that USB being packet-based does not generate interrupts
upon receiving traffic and may be fundamentally rate limited for
one-character-at-a-time operation. My question for you (or someone else you
might be able to point me to) is whether or not such a device can be programmed
to poll often enough to not miss our timing window. I have enabled low-latency
and done everything I can think of, short of digging into driver code.
For reference, I've tried this on RedHat Enterprise Linux with kernel
2.6.18-194.32.1.el5 and on my Gentoo laptop with 2.6.38 kernel and the device
is a VSCOM 4 port RS482/485 using FTDI FT232 series UARTs.
Any advice? If there is no solution here with the USB device (as I
suspect) can you recommend any PCI boards that may work better for my
application?
Thanks in advance for any help you may be able to offer.
Walter
P.S.: Please make sure to cc me in any response.
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[*] For a USB device, data transfer is done in packets. If data is to be sent
from the PC, then a packet of data is built up by the device driver and sent to
the USB scheduler. This scheduler puts the request onto the list of tasks for
the USB host controller to perform. This will typically take at least 1
millisecond to execute because it will not pick up the new request until the
next 'USB Frame' (the frame period is 1 millisecond). Therefore there is a
sizable overhead (depending on your required throughput) associated with moving
the data from the application to the USB device. If data were sent 'a byte at a
time' by an application, this would severely limit the overall throughput of
the system as a whole.
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