RS232 to RS485

[The Master]

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

[The Master]

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

[dhouston]

You're now an expert who can help the next RS485 noob.

[The Master]

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

[skimask]

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

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...

[The Master]

It "could happen" so it probably will the first time i try it. Im almost out of CAT5 cable so i guess it wouldnt hurt to buy another box and test it over the full length (about 300m). Ive just been reading a page about some of the things that can go wrong but i think ive got most of them covered. There might be a problem if the main box isnt connected because none of the lights bias the line themselves. I think thats a good thing because aparently too many devices biasing the line can stop data from being transmitted. Once the main box is connected they all need turning off and back on again before they can work anyway.

Another idea is to put the biasing resistors in the terminating plug. Its possible because once the lights are turned on there will be both positive and negative voltages coming out of the socket. I doubt ill need to do that though

I would like to connect up way more MAX485's than the specification allows and see how many it takes to stop it working just to make sure everything will be fine. I think the limit was about 30 unless i have those other ones that allow 128. I might need a bigger breadboard for that :P