Found sometime to go over this in details and here're are my findings, which may be not be correct at all, so please hint out the problems.
Let's start with a perfect baud rate match between the transmitter and receiver:
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Hopefully the image is self explanatory. I have assumed the receiver can poll the line status/voltage many times faster than the baud rate, to simplify things (I read about programming a PIC to handle 115200 baud rate with 4Mhz clock). That's why there is a solid gray area which shows constant monitoring of the line by the receiver. I've also assumed perfect vertical rising/falling edges.
Now here's where the receiver is running at a faster baud rate than the transmitter. This is what I think the maximum difference between the two baud rates can be:
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Note that when receiver polls the line in where it expects the middle of the stop bit to be, it's just after the rising edge of the actual stop bit. If you stretch out the bottom wave any further, the receiver will read in the 8th data bit instead of the stop bit.
However, staying within the range shown in the graph, there will be no errors even in back to back transmission, since the receiver syncs to the start bit for each byte.
In situations where the baud rate of receiver is less than that of the transmitter, I'm not quite sure what would happen. If we assume that the receiver waits for the entire length of stop bit before polling the line for the next start bit, there is no way the receiver can keep in sync in back to back transmissions, unless the baud rates differ only by less than half a bit!
However, if we assume that the receiver will start polling the line for the next start bit right after checking for the stop bit (which is half way through it), then:
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Putting the two situations together, I came up with this formula to find out if two different bit rates can work without errors in back to back transmissions:
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This formula only works for a 8 bit data, 1 stop bit settings.
And here's a graph of it:
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The red shaded areas are the safe areas.