Hi all,

I have to implement a very simple multidrop RS485 application,
carrying slow 2400bps or even 1200bps data from a PC to a half-dozen
devices about 300 feet away. The data flow is unidirectional; the
devices don't talk back. Low cost is a significant factor in this. I
plan to stick an RS485 driver on an AVR, weld on the small additional
logic I need, and put it in a box.

Two quickies:

1. What's the cheapest assembled RS232 to RS485 converter I can
recommend to the customer? (I'm not supplying this piece to him. I am
going to test with an SBC that has RS485 built-in).

2. Can I use cat5 network cable for the cable run? I ask because both
I and the customer have metric truckloads of the stuff lying about.

In article <6...@posting.google.com>, "Lewin A.R.W.
Edwards" <l...@larwe.com> wrote:


> Hi all,
> I have to implement a very simple multidrop RS485 application, carrying slow
> 2400bps or even 1200bps data from a PC to a half-dozen devices about 300
> feet away. The data flow is unidirectional; the devices don't talk back. Low
> cost is a significant factor in this. I plan to stick an RS485 driver on an
> AVR, weld on the small additional logic I need, and put it in a box. Two
> quickies:
> 1. What's the cheapest assembled RS232 to RS485 converter I can recommend to
> the customer? (I'm not supplying this piece to him. I am going to test with
> an SBC that has RS485 built-in).

I'm no help on this one.


> 2. Can I use cat5 network cable for the cable run? I ask because both I and
> the customer have metric truckloads ofthe stuff lying about.

For lowly 2400 baud, cat5 should perform well. I could reliably run close to a
mile at 9600 baud with much worse than cat5. I used the 1/256th load 485
drivers though.

On 29 Apr 2004 10:31:04 -0700, l...@larwe.com (Lewin A.R.W. Edwards)
wrote:

>1. What's the cheapest assembled RS232 to RS485 converter I can
>recommend to the customer? (I'm not supplying this piece to him. I am
>going to test with an SBC that has RS485 built-in).

RS-232/485 converters usually contain some logic for flow direction
control, so my guess is that a simple RS-232/422 converter might be
cheaper, just don't use the Rx pair. 

At those speeds and distances even RS-232 might work, if you do not
have to worry about ground potential differences. However, if there
are potential differences, standard RS-422/485 chips work with at
least 5 V ground potential differences without galvanic isolation.

>2. Can I use cat5 network cable for the cable run? I ask because both
>I and the customer have metric truckloads of the stuff lying about.

At those speeds and distances, this should not be a problem.

Paul

"Lewin A.R.W. Edwards" <l...@larwe.com> wrote in message
news:6...@posting.google.com...
> Two quickies:
>
> 1. What's the cheapest assembled RS232 to RS485 converter I can
> recommend to the customer? (I'm not supplying this piece to him. I am
> going to test with an SBC that has RS485 built-in).

I use http://www.kksystems.com. I have seen cheaper, I think, but not much.
Also KK Systems are an old customer of mine, and I'm loyal ;).

> 2. Can I use cat5 network cable for the cable run? I ask because both
> I and the customer have metric truckloads of the stuff lying about.

Yes, no problem.

Steve
http://www.sfdesign.co.uk
http://www.fivetrees.com

"Lewin A.R.W. Edwards" <l...@larwe.com> ???
news:6...@posting.google.com ???...
> Hi all,
>
> I have to implement a very simple multidrop RS485 application,
> carrying slow 2400bps or even 1200bps data from a PC to a half-dozen
> devices about 300 feet away. The data flow is unidirectional; the
> devices don't talk back. Low cost is a significant factor in this. I
> plan to stick an RS485 driver on an AVR, weld on the small additional
> logic I need, and put it in a box.
>
> Two quickies:
>
> 1. What's the cheapest assembled RS232 to RS485 converter I can
> recommend to the customer? (I'm not supplying this piece to him. I am
> going to test with an SBC that has RS485 built-in).
>
try http://www.qkits.com/serv/qkits/qkitscat/order.asp?Code=KC485


> 2. Can I use cat5 network cable for the cable run? I ask because both
> I and the customer have metric truckloads of the stuff lying about.
>
It should be fine. Remember to pick the correct pair to get optimum
performance.

In article <c6s90v$m...@imsp212.netvigator.com>, Vincent Lai wrote:

>> 2. Can I use cat5 network cable for the cable run? I ask because both
>> I and the customer have metric truckloads of the stuff lying about.
>
> It should be fine. Remember to pick the correct pair to get optimum
> performance.

OK, I give, which one is the correct pair?  ;)

-- 
Grant Edwards                   grante             Yow!  If Robert Di Niro
                                  at               assassinates Walter Slezak,
                               visi.com            will Jodie Foster marry
                                                   Bonzo??

"Lewin A.R.W. Edwards" <l...@larwe.com> wrote in message
news:6...@posting.google.com...
> Hi all,
>
> I have to implement a very simple multidrop RS485 application,
> carrying slow 2400bps or even 1200bps data from a PC to a half-dozen
> devices about 300 feet away. The data flow is unidirectional; the
> devices don't talk back. Low cost is a significant factor in this. I
> plan to stick an RS485 driver on an AVR, weld on the small additional
> logic I need, and put it in a box.
>
> Two quickies:
>
> 1. What's the cheapest assembled RS232 to RS485 converter I can
> recommend to the customer? (I'm not supplying this piece to him. I am
> going to test with an SBC that has RS485 built-in).
>
> 2. Can I use cat5 network cable for the cable run? I ask because both
> I and the customer have metric truckloads of the stuff lying about.

This site has a converter for £32 but you'll also need an external power
supply.
http://www.bb-europe.com/product.asp?dept%5Fid=3&sku=485CON

Cat 5 is fine for RS485, you should be able to get many times further than
300ft.

Peter


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moocowmoo wrote:
> "Lewin A.R.W. Edwards" <l...@larwe.com> wrote in message
>>
>> I have to implement a very simple multidrop RS485 application,
>> carrying slow 2400bps or even 1200bps data from a PC to a 
>> half-dozen devices about 300 feet away. The data flow is
>> unidirectional; the devices don't talk back. Low cost is a
>> significant factor in this. I plan to stick an RS485 driver on
>> an AVR, weld on the small additional logic I need, and put it
>> in a box.
>>
>> Two quickies:
>>
>> 1. What's the cheapest assembled RS232 to RS485 converter I can
>> recommend to the customer? (I'm not supplying this piece to
>> him. I am going to test with an SBC that has RS485 built-in).
>>
>> 2. Can I use cat5 network cable for the cable run? I ask because
>> both I and the customer have metric truckloads of the stuff
>> lying about.
> 
> This site has a converter for £32 but you'll also need an
> external power supply.
> http://www.bb-europe.com/product.asp?dept%5Fid=3&sku=485CON
> 
> Cat 5 is fine for RS485, you should be able to get many times
> further than 300ft.

Try the raw RS232 signal.  We used to routinely connect terminals
over 1000 feet or more of unshielded multi-wire cable (not twisted
pairs) for 2400 baud operation with no noticeable problems.  Of
course the RS232 signals met RS232 specs in the first place (i.e.
+-12V delivered).

-- 
Chuck F (c...@yahoo.com) (c...@worldnet.att.net)
   Available for consulting/temporary embedded and systems.
   <http://cbfalconer.home.att.net>;  USE worldnet address!

> >> 2. Can I use cat5 network cable for the cable run? I ask because both
> >
> > It should be fine. Remember to pick the correct pair to get optimum
> 
> OK, I give, which one is the correct pair?  ;)

I read this message to mean "pick two wires that form a twisted pair,
don't arbitrarily choose one wire from one pair, and the other wire
from a different pair" :)

On Fri, 30 Apr 2004 10:04:41 GMT, CBFalconer <c...@yahoo.com>
wrote:

>moocowmoo wrote:
>> "Lewin A.R.W. Edwards" <l...@larwe.com> wrote in message
>>>
>>> I have to implement a very simple multidrop RS485 application,
>>> carrying slow 2400bps or even 1200bps data from a PC to a 
>>> half-dozen devices about 300 feet away. The data flow is
>>> unidirectional; the devices don't talk back. Low cost is a
>>> significant factor in this. I plan to stick an RS485 driver on
>>> an AVR, weld on the small additional logic I need, and put it
>>> in a box.

>Try the raw RS232 signal.  We used to routinely connect terminals
>over 1000 feet or more of unshielded multi-wire cable (not twisted
>pairs) for 2400 baud operation with no noticeable problems.  Of
>course the RS232 signals met RS232 specs in the first place (i.e.
>+-12V delivered).

The RS232 might work well in the OP's case, but especially if the
boxes are in a different buildings, then ground potential issues needs
to be handled. 

Keeping the boxes floating usually solves the problem, i.e. put the
devices in plastic boxes and feed them with individually with wall
warts (or with batteries if these are low consumption boxes). Connect
the signal ground only to the data cable. With a grounded PC, there
should not be any electrostatic charge buildup.

Paul