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Hi, In Modbus protocol, RTU mode, use 3.5 charater silent time as a message start or end in current BPS, how can i detect this interval, use a timer or ...? (the mcu i use is at89c52, timer0 is used as a 1ms timer interrupt, timer1 use for baud rate generator)
Hi, Liu. I'm looking for the same information. Did you find it? If you did, could you please send to me, 'cause I do'nt know how to do this on 89C52. Thanks, Giovani Guidolin
Don't most '52 derivatives have a Timer2?
There's the suggestion in http://www.keil.com/forum/docs/thread3738.asp about timestamping the last event. For fairly low serial rates, the 1ms timer tick would do. (That's about 1 character time for a 9600 bps UART.) The Modbus specs are "at least" 3.5 character times, I think, so being too slow only hurts throughput rather than correctness. Once the data rate hits about 40 kbps, this will start to be a factor. Perhaps you could use the BRG timer to measure 4 character silent intervals (when it's not in use as a BRG)? You'll have to be able to switch the timer mode in half a character time, which again shouldn't be too bad at low data rates. It should be only a dozen or so instructions. Depending on what else your application has to do, a delay loop might suffice. You could code one that watches the timer value for that 1ms timer tick. Search the forum for a number of good threads on ways to code delay loops.