Lpc2000

Xtian Xultz wrote:

>>Ha!  Welcome to RS485.  It is a bit more complex than it looks.
>>
>>I don't know why you believe that you needs such precise control over
>>the relationship of TXD/RXD and the DIRection?  By it's very nature,
>>RS485 communications over twisted pair require that the line be
>>precharged before it can be used.
>>
>>Think about it, the wire is floating electrically, any stray magnetic or
>>electrical field can induce a blip into the "dead" wire.  By first
>>turning the direction to Transmit, then hold that for a few TX char
>>times, it allows the uart at the RX side to settle down.  The uart that
>>the RX side may have heard a blip and is in the process of receiving a
>>erroneous char.  Once you precharge the line, send your data, then HOLD
>>the carrier (direction) until the last char has been sent and then flip
>>the line back to RX mode.
>>
>>Most of the systems I've seen / worked on have a "quiet" line floating
>>as the units are all listening for some other unit to speak.
>>
>>TomW
>>    
>>
>
>Hello Tom.
>I am right now working with RS485. I had most of the problems (the last 3 
>days) with a good way to make the computer transmit over RS485. I tried with 
>RTS controling the DE pin, but it was a nightmare. I am drawing a board right 
>now with a 555 that is trigged by RTS and every low level on the TX line to 
>control the state of DE.
>
>  
>
No, no, don't even try that!  Go with the RTS Line, it is the most 
accepted practice.  Once you build your first driver code, you can move 
it to other uarts as most uarts have an RTS control line.  I agree, it 
is not simple to do RS485 with the line precharge requirement, but you 
should be able to come up with a fairly simple solution...

Like, have the software that puts the TX data into the data queue hold 
off on stuffing the first char into the uart TX buffer.  Have it set a 
count value to a preset value, then set the LINE to TX directions.  
Everytime a new char is put into the buffer, preset the counter.  No 
data is put into the TX uart register yet.

Have a timer try to decrement the counter, if the counter is non-zero, 
decrement it.  If the decrement goes to zero, take a SPECIAL(!) char / 
pair of them, and send stuff these into the TX uart data register to 
start the interrupt system into motion.

I said SPECIAL char(s) as I qualify the beginning of the transmission 
with a series of 0xfe chars.  The RX side looks for at least one 0xfe at 
the start of each data block.  The leading 0xfe's are peeled off and 
discarded.

Oh, BTW, you will need to checksum that data block somehow.  You need to 
verify that what you are recieving is not noise.  When you have a pair 
of wires running several miles / kilometers, you are going to start 
hearing Terrestial Noise.  Noise from magnetic field flux changes (e.g. 
"northern lights"), noise from lightning storms in the area, also from 
distant storms.

A lightning stroke that is in the local area (within several miles) will 
induce from 3volts to 30volts per foot of wire.  THAT is a lot of 
energy!  You need a way to clamp this, don't bother with MOVs or PTCs, 
they are too slow and degrade with each use:  try the classic PI 
arrangement of Transil (sub-ns Zener), resistor, Transorb (gas tube).


>You talked about a precharge, do you know what are a safe time to precharge 
>the line?
>
>
>  
>
At least one and a half character time, compute your char time based on 
baudrate.  Typically, I will use 10ms precharge and 5ms trailing delays 
at 1200 baud.  BTW, 1200 baud may be slow, but it can go a really long 
distance over RS485, typically 5-7 miles (and longer with an end-user 
who cannot RTFM).


TomW


-- 
Tom Walsh - WN3L - Embedded Systems Consultant
http://openhardware.net, http://cyberiansoftware.com
"Windows? No thanks, I have work to do..."
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