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Hi, Ive been testing this RS485 stuff out tonite. I had bypassed it before to get everything else working. Now ive tried to get the RS485 bit working and ive found a little problem. The first 1 or 2 bytes seem to get missing from the data. I think its because the 555 doesnt react quick enough to the data. Im using 115200 baud. Is there anything i can do about this problem?
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So what your saying is scrap the 555 idea and go back to using RTS. I guess that is an option. Ill have to look into how that will affect the software. I know i can easily control RTS in VB using the comm control but i do like to use API for speed. Ive not had the need to use handshaking before so i should probs read up on that.
Thanx for the suggestion and the diagram. Now i see the point in the MAX232 having 2 ins and outs
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Why not sending some control characters then wait for the start character, dump it and get the next.
Like this:
Send "#######$data"
on PIC side:
Hserin [wait("$"),str mydata\data_bytes]
Ioannis
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I suppose thats a possibility. Im using the hardware UART so its a little harder but still possible.
Is there anything faster than a 555? What about bypassing it? I noticed the output from the MAX232 chip seems to be inverted but if i could invert it again and connect it and the 555 through resistors to the MAX485 then it should be a lot faster because it doesnt rely on the 555 to turn it on, the 555 will simply keep it turned on for a short while after the data stops.
I guess the only problem now is that the MAX485 chip might be too slow to react too
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I have used the 485SD9TB converter from BB Electronics with no problems.It does exactly what you are trying to do.Its not cheap,but you can check out the schematic and maybe reproduce the circuit.
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Ive tried the RTS method. It takes quite a while to turn on using software. Im only talking a few ms but its still enough to cause problems.
I think a circuit based on that scematic would be best. I dont have any logic gates that i can make a NOT gate out of so i cant test it yet.
I will have a go at the control characters method. That one seems like it should definately work aslong as i can program it properly. At the moment it relies on counting the bytes that arrive. With the control characters 1 or more will be missing.
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I doubt very much whether it's the 555. That method is widely and successfully used for automatic control of line direction. The fact that subsequent bytes are OK would also seem to indicate it's not the 555. Are you using termination resistors? They are absolutely necessary at 115200.Originally Posted by The Master
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I am using a termination resistor at the end with the 555 aswell as 2 biasing resistors exactly as shown in the diagram in your first post. I have the RS232 to RS485 converter plus 2 lights on breadboards at the moment. The MAX485 chips are less than 5cm apart but i can add another terminating resistor at the end just to be sure.
Ive been thinking about this one and i think i got it the wrong way round. The thing that got me thinking is that the offset is constant even if i stop sending data then start again. You would expect it to keep shifting one or 2 bytes further out everytime the 555 gets chance to turn back off. I looked at how the circuits reacted to the data and it seems they are actually getting an extra byte or 2. I think that may be to do with the way ive programmed it but now im looking in the right place i have a better chance of solving the problem.
I do like the idea of using a 555 because it allows me to have a status light that doesnt flash too quick to see and doesnt need any other components
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It turns out it was a programming problem. As the chips start up the first one sends out a byte to tell the others how many chanels it has then goes into receive mode. The second one then does the same thing but the byte it sends out gets counted as data by the first light. Im really not sure why it was sometimes 2 extra bytes but ive reprogrammed it and it seems to be working perfect now. Sorry for wasting your time
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Haha, I wouldnt say 'expert'. Remember im the one who just got it wrong. I havnt tested with long wires yet so no doubt ill have more problems
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If you're using drivers designed for '485 and you design the circuit within the '485 specs, why would you have problems with long wires? Shouldn't... Could happen...but shouldn't...
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