Two quick RS-485 questions

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

http://www.2cmc.com/pga/pga_2h.htm

>
>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.

my 2c worth

> >> 1. What's the cheapest assembled RS232 to RS485 converter I can
> >> recommend to the customer? (I'm not supplying this piece to
> 
> Try the raw RS232 signal.  We used to routinely connect terminals
> over 1000 feet or more of unshielded multi-wire cable (not twisted

I can't be sure the computer in question will meet RS232 specs, but
more importantly, I was asked specifically to implement RS485 in this
case. Also there are actually to be two installations - 300ft is the
run length for the first installation, but the second installation
"could be longer" (they aren't clear on this yet, it's in a different
facility a long way away).

This application is basically a remote indicator panel; there is a
computer in an office that needs to drive a large matrix of indicator
lamps sitting next to a conveyor belt on a warehouse floor. Actually
there are five conveyor belts, and each has a bank of approximately
five hundred indicator lamps next to it. It sounds very "Metropolis"
because there is apparently a worker watching the indicators intently;
as a "something" comes down the conveyor, the worker grabs something
as indicated by a lamp on this panel, and sticks it onto the
"something".

I have to admit I don't fully understand how the workflow is supposed
to function in this environment, but I was given very detailed
specifications on how this particular piece is supposed to work, so I
am kind of ignoring the big picture.

On 2004-04-30, Lewin A.R.W. Edwards <larwe@larwe.com> wrote:
>> >> 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" :)

Yes, I'm sure that's what he meant.  

OTOH, yellow is the fastest color on cars[1], so perhaps there's a
best color for RS-485 signalling.

[1] I learned that at http://www.riceboypage.com/. 

-- 
Grant Edwards                   grante             Yow!  Yow! I threw up on
                                  at               my window!
                               visi.com            

Thanks muchly to all who replied. I am in good shape to get this
project completed. It's a fun little item. And the news that I can
maybe run as much as several thousand feet on cat5 is great, because I
still have no info on that second site.

"Lewin A.R.W. Edwards" <larwe@larwe.com> wrote in message
news:608b6569.0404290931.33c27d32@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).

I think a current loop might be your cheapest option. You would need a
proper 'driver' at the TX side while all receivers are merely cheap
optocouplers at a fraction of the price of an RS-422/485 receiver. And as a
bonus, you'll get galvanic isolation for free.

> 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.

With 2400 bps and a current loop you could even use fence wire. So yes, cat5
will be fine :-)

Meindert

> > I have to implement a very simple multidrop RS485 application,
> > carrying slow 2400bps or even 1200bps data from a PC to a half-dozen
> 
> I think a current loop might be your cheapest option. You would need a
> proper 'driver' at the TX side while all receivers are merely cheap

Ah, but I'm required by the customer to use RS485, sorry. I should
have made that clearer in my original posting.

BTW, the driver/receiver I'm using is the SN75LBC179P, which is $1.26
in 25pc quantities, not too bad. I just finished a nasty prototype PCB
layout and created the BOM, and I find that as long as they make at
least 10 of these, it's going to be _way_ cheaper than my ballpark
guess. Since they already have purchase approval for the ballpark
guess, I come out looking like a hero for being under budget ;)

"Lewin A.R.W. Edwards" <larwe@larwe.com> wrote in message
news:608b6569.0405030258.363074a8@posting.google.com...
> > > I have to implement a very simple multidrop RS485 application,
> > > carrying slow 2400bps or even 1200bps data from a PC to a half-dozen
> >
> > I think a current loop might be your cheapest option. You would need a
> > proper 'driver' at the TX side while all receivers are merely cheap
>
> Ah, but I'm required by the customer to use RS485, sorry. I should
> have made that clearer in my original posting.
>
> BTW, the driver/receiver I'm using is the SN75LBC179P, which is $1.26
> in 25pc quantities, not too bad.

That is indeed cheaper than the Maxim counterpart.

> I just finished a nasty prototype PCB
> layout and created the BOM, and I find that as long as they make at
> least 10 of these, it's going to be _way_ cheaper than my ballpark
> guess. Since they already have purchase approval for the ballpark
> guess, I come out looking like a hero for being under budget ;)

:-)

Meindert

If you want to build your own converter, I recommend MAX485 or MAX487. 
Not the cheapest parts, but one of the best I think.
We are using MAX487 with 92 workloads and get 1200 meters with 115200 
Baud and CAT5 without problems. Not bad I think.

> I can't be sure the computer in question will meet RS232 specs, but
> more importantly, I was asked specifically to implement RS485 in this
> case.

Almost all "real" PC's use at least +/- 9V, which is enough. Be aware if 
you  are using Notebooks, the often use +/-5V or less (especially when 
powered from the batteries).


The simplest an fastest way is to buy a RS232-RS485 converter box.
But there are many many converters available, and many of them are rubbish.
We used the IPCON I-7520R over 100 times and got 1800m with 9600 Baud 
and CAT5.


Best regards,

-- 
Bernhard Roessmann

"Meindert Sprang" <mhsprang@NOcustomSPAMware.nl> writes:

> "Lewin A.R.W. Edwards" <larwe@larwe.com> wrote in message
> news:608b6569.0405030258.363074a8@posting.google.com...
> > > > I have to implement a very simple multidrop RS485 application,
> > > > carrying slow 2400bps or even 1200bps data from a PC to a half-dozen
> > >
> > > I think a current loop might be your cheapest option. You would need a
> > > proper 'driver' at the TX side while all receivers are merely cheap
> >
> > Ah, but I'm required by the customer to use RS485, sorry. I should
> > have made that clearer in my original posting.
> >
> > BTW, the driver/receiver I'm using is the SN75LBC179P, which is $1.26
> > in 25pc quantities, not too bad.
> 
> That is indeed cheaper than the Maxim counterpart.

Cheapest is probably a generic 75176, e.g.  DS75176BM or SN75176AD, ~ $0.20
each / 100.

-- 

John Devereux

John Devereux wrote:

> 
> Cheapest is probably a generic 75176, e.g.  DS75176BM or SN75176AD, ~ $0.20
> each / 100.
> 

Where do you get new ones for that price? Cheapest I have seen is double 
that.

Thanks,
Joe Legris