AVR-Chat

Peter Jakacki wrote:
> Russell Shaw wrote:
> 
>>I run rs232 from my workroom to the garage (30m).
> 
> I'm sorry to hear that, I'd do the same if I had nothing else. I would 
> use RS-422 (not 485) "IF" I needed higher baud rates. The cables are 
> usually 4-core + shield so I would not be at any loss. Termination is 
> really only needed for high-speed operation as the reflections have 
> negligible effect at RS-232 speeds anyway. I've run kilometres of 
> cabling in skyscrapers with simple multi-node 19.2K RS-485 networking 
> over untwisted fire-proof cabling without the "need" for termination 
> even though the cables were not in any ordered configuration. Impossible 
> with RS-232.

Non-termination of such long lengths and of uncontrolled impedance
characteristics is asking for trouble. I'd only do that if i verified
the received waveform with a cro.

>>At my previous job, rs232 was run from the lab to various measuring equipment
>>outside (10-20m).
>>
>>I know of several radio hams that run rs232 from their radio room to things
>>outside and up radio towers (10-30m).
>>
>>RS232 was designed for unterminated lines which keeps things simple,
>>especially for multi-drop star networks. RS485 requires a single backbone
>>with terminator.
> 
> RS-232 is not multidrop and at the low speeds it operates at would not 
> need termination.

There are tristate rs232 chips for doing multidrop.

>>RS232 can be fed thru radio coax and separated out with a filter, unlike
>>rs485 which requires a balanced line.
> 
> Cool, I like that, but is it really real RS-232 anymore? (I think not)

Using RF chokes and isolating caps, the coax looks like normal wire.

>>http://www.rs485.com/rs485spec.html
>>
>>RS485 requires three wires but rs232 only needs two.
> 
> Place a 1.5V battery across the balanced line to generate a signal, then 
> reverse it to invert the signal. Did it require a separate ground? No! 
> It is out of ignorance that a ground is connected, it is simply provided 
> either for additional shielding or for kludging a particularly bad 
> ground loop problem. By powering the remote equipment from the line it 
> obviates the need for isolation.

This is the root cause of unreliable comms.

                  long wire-pair
Box_A <======================================> Box_B
   |                                              |
   | Each box 3kohms common-mode Z to ground      |
   |                                              |
  --- case-ground       Several square           ---
  --- capacitance       metres loop area         ---
   |                                              |
   |   conductive ground/soil/floor/wiring        |
--+----------------<-Ignd->----------------------+--
                      Ignd: ground-current

Your receivers are highly vulnerable to having their common-mode
voltage range exceeded by voltage drop caused by ground-current,
and loop voltage induced by fluctuating magnetic field within
the loop area. Higher-frequency noise will have more effect
because the case capacitance impedes less and the loop voltage
is proportional to magnetic field rate of change. It is highly
susceptable to long-distance lightning interference.

Ground wire *must* be used for reliability.

> Anyway, I like the fact that you are not strictly abiding by the 
> standard and doing what engineers do best .... tinkering ;)

I always work with known and bounded parameters. Never "tinker".