AVR-Chat

Russell Shaw wrote:
> 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.
cro??? Of course I verified it with a scope and though I didn't "need" 
termination I still used it.
>
>>> 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.
Please, I am fairly sure the RS-232 standard does not say multi-drop. I 
have implemented multi-drop RS-232 in the distant past but I have better 
things now. Read what Maxim themselves say (they must know I guess).

"Typical RS-232 transmissions seldom exceed 100 feet for two reasons. 
Firstly, the difference between transmitted levels (±5V) and receive 
levels (±3V) allows only 2V of common-mode rejection. Secondly, the 
distributed capacitance of a longer cable can degrade slew rates by 
exceeding the maximum specified load (2500pF). Because the RS-232 was 
designed as a point-to-point rather than multidrop interface" 
(http://www.maxim-ic.com/appnotes.cfm/appnote_number/723)
>
>>> 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.
I have done similar things myself using inductors and caps to isolate 
signals but I like the way amateurs improvise according to knowledge 
without trying to rigidly adhere to standards for the sake of standards 
(no longer RS-232).

>
> 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.
No, you are assuming that current will flow, I do design professional 
equipment for a living, not just for my garage. If my equipment is not 
line-powered then I like to opto-isolate the 485. How can current flow 
in these two situations??
>
>> 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".
Call it what you will, this is like the chicken and egg thing. How do 
you know and bound parameters unless you "tinker" in the first place????


*Peter*