On 1/20/2011 9:42 PM, James Y Knight wrote: > On Jan 20, 2011, at 6:32 AM, Albert Brandl wrote: >> The main problem seems to be the specification of Modbus RTU. RTU >> messages are separated by time gaps that are at least 3.5 characters >> long. Unfortunately, this is the only reliable way for separating >> messages - there is no length header or something similar. The >> specification does define a trailing CRC, though. > > I don't think you'll ever be able to do this reliably in Python. At 9600 > baud, 3.5 characters is just about 275hz (9600/10/3.5). Linux schedules > process at around 500hz. So, unless you use real-time scheduling, I think you > have no guarantee that you'll be able to detect such a gap at all -- some > other process may be running and you may completely miss it. And you can't > reasonably use real-time scheduling in python... > > On the other hand, are you sure you need this? http://www.libmodbus.org/ > seems to have code which figures out what length packets should be, and no > code to detect a 3.5char delay between packets. That seems much more reliable. > > It also doesn't seem to have any code to ensure a 3.5char between sent > packets, which indicates to me that either it's totally brokem or else that > not even the hardware end actually uses a 3.5char delay to determine packet > boundaries...I'd guess the second, since even on an embedded CPU, it's much > trickier to ensure the appropriate serial port timing than to simply count > the correct number of bytes.
Building upon James' reply a little further, I recently did a ModBus RTU implementation on a microcontroller and you need both a 1.5 and 3.5 character timer for correct operation. With the micro, I used hardware timers with uS resolution synchronized to incoming characters via the UART interrupt. As James mentions, user-space Python is simply too far from the hardware to do this sort of thing. You'd probably have to write a custom UART driver and run it on a RTOS to get correct timing. That being said, you can probably write something that will work 99.9% of the time, but may not respond correctly for some weird corner case scenarios. Jason Valenzuela _______________________________________________ Twisted-Python mailing list Twisted-Python@twistedmatrix.com http://twistedmatrix.com/cgi-bin/mailman/listinfo/twisted-python
