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'RS485 question'
1998\03\08@031345 by juliusz

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Hi

I am student of the faculty of Electronics in Poland and want to build RS232 to RS485 converter in order to use it as my home LAN extension via one twisted pair wire to my friend's home for Internet purpose. He lives 1000 meters from me and I think RS485 will be god for 115200bps. It is easy to buy it but I want to build myself. We will use Linux (as always).

Hardware is very clear for such purpose but I can't find information how to convert RXD and TXD ? two signals into 1 signal of the current loop. I mean I don't know how to create protocol between my 485 modems? Are RXD and TXD multiplexed (switched) every transmitted/received byte like

RXD TXD RXD TXD ....

Or maybe RTS/CTS signals must be used to switch between receive/transmit mode and PC software (like port driver) takes care of it?
But what will happen for instance during initial negotiation when both computers will want to transmit or receive simultaneously? It must be simple null modem but based on RS485. No drivers will not be necessary.

I write software in Assembly language and I have built some good controllers and measuring devices with AD converters, displays etc.. I use PC everyday but such problem made me crazy.
I know that my problem is very funny, and basic in general but please give me some piece of information or URL to find it?

Please help

Juliusz

1998\03\08@090934 by Alexandre Guimaraes

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   Use four wires, one pair for TX and another pair for RX. This way you
have full duplex and do not have to worry about switching at all. In the
software side you can use it as a regular RS-232 without handshake.

Best regards,
Alexandre Guimaraes
Microset Eletronica Ltda
spam_OUTalexgTakeThisOuTspamiis.com.br


> Hardware is very clear for such purpose but I can't find information how
to convert RXD > and TXD ? two signals into 1 signal of the current loop. I
mean I don't know how to
> create protocol between my 485 modems? Are RXD and TXD multiplexed
(switched) every
> transmitted/received byte like
>
> RXD TXD RXD TXD ....

1998\03\08@105900 by Mark Lezama

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Hello Julius, In RS485 you need something that take control of the transmision
becouse is half duplex (transmision and recepcion but not at the same time), so
I think in your case is better use RS-422. In RS422 your problem is how much is
the cable cost, becouse you need 2 times more cable than the RS485.

BTW, for convert a RS232 signal to RS485 signal you will need a MAX232 (or
something like that) and SN75176B (there more and best from MAXIN). I recoment
you use a optocupler between RS232 and RS485 becouse you need take care of your
computer, I think. Well, To tell the true I have that you need if you are
interesting contact me directly and I will send you the
schematic and If you have EAGLE the board too.

Mark Lezama.

juliusz escribis:

> Hi
>
> I am student of the faculty of Electronics in Poland and want to build RS232
to RS485 converter in order to use it as my home LAN extension via one twisted
pair wire to my friend's home for Internet purpose. He lives 1000 meters from
me and I think RS485 will be god for 115200bps. It is easy to buy it but I want
to build myself. We will use Linux (as always).
>
> Hardware is very clear for such purpose but I can't find information how to
convert RXD and TXD ? two signals into 1 signal of the current loop. I mean I
don't know how to create protocol between my 485 modems? Are RXD and TXD
multiplexed (switched) every transmitted/received byte like
>
> RXD TXD RXD TXD ....
>
> Or maybe RTS/CTS signals must be used to switch between receive/transmit mode
and PC software (like port driver) takes care of it?
> But what will happen for instance during initial negotiation when both
computers will want to transmit or receive simultaneously? It must be simple
null modem but based on RS485. No drivers will not be necessary.
>
> I write software in Assembly language and I have built some good controllers
and measuring devices with AD converters, displays etc.. I use PC everyday but
such problem made me crazy.
> I know that my problem is very funny, and basic in general but please give me
some piece of information or URL to find it?
>
> Please help
>
> Juliusz

1998\03\10@064952 by maxruben

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face
> Hi
>
> I am student of the faculty of Electronics in Poland and want to build RS232
to RS485 converter in order to use it as my home LAN extension via one twisted
pair wire to my friend's home for Internet >
> Hardware is very clear for such purpose but I can't find information how to
convert RXD and TXD ? two signals into 1 signal of the current loop. I mean I
don't know how to create protocol between my >
> RXD TXD RXD TXD ....
>
> Or maybe RTS/CTS signals must be used to switch between receive/transmit mode
and PC software (like port driver) takes care of it?
> But what will happen for instance during initial negotiation when both
computers will want to transmit or receive simultaneously? It must be simple
null modem but based on RS485. No drivers will not >
> I write software in Assembly language and I have built some good controllers
and measuring devices with AD converters, displays etc.. I use PC everyday but
such problem made me crazy.
> I know that my problem is very funny, and basic in general but please give me
some piece of information or URL to find it?
>
> Please help
>
> Juliusz
>

Hello Juliusz

As somebody already has pointed out to you RS485 is only half
duplex with 2 wires (1 pair). To make it full duplex use 4 wires
(2 pairs). If there are only 2 nodes this is the same as two
RS422 connections.

Also, you must have some kind of reference wire connected between
the two nodes so the voltage difference between the inputlines and
the nodes ground doesn't get too big. This can be the shield of
cable. It might be a good idea to isolate the RS422/485 side of the
nodes with an isolated DC/DC converter and optocouplers.

--------------------------------
Ruben Jvnsson
.....maxrubenKILLspamspam@spam@mail.bip.net
--------------------------------

1998\03\10@113520 by Keith Howell

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Regarding Juliusz's problem, what is the application?

If you want to have an internet link to your friend's house I presume
the distance is a bit too far for reliable RS232. You could simply
buffer the signals to RS422/485 levels (5V differential) and run
internet's SLIP (Serial Line Interface Protocol).

If you want to halve cable cost by running in half-duplex mode, then can
SLIP cope with this? If not you'll have to write software that can.
If it's a one-off, it is not worth the human time to do.

Neither is re-inventing one-off control networks. It's been done for
factory control systems (called fieldbuses). The biggest problem with
them is that there are so many of them that nobody dares commit
factories to a standard incase it does not become _the_ most popular
standard.

Having spent some professional design time with these networks, I can
say that many used half-duplex RS485 as the physical layer. To do it
right to spec, you need low-capacitance twisted pair, shielded
(factories are electrically noisy) and preferably with a drain wire. The
latter you connect to the shielding cases of nodes via a 100nF/1M
parallel RC network. This shorts HF AC noise, and drains DC signals like
static electricity without providing a potentially lethal low-impedance
path. Most economically practical cables are simplytwisted pairs with
shield - no ground ref to connect. You can get round this by having say
a 100R from you buffer's isolated ground to the cable shield.

The RS485 bus needs 75R terminations at either end if you intend to run
it at worthwhile speeds. Node buffers are usually SN176 types. Buffers
are usually in  listen' mode (output hi-z). Nodes have intelligence to
enable their transmitters at approriate times. Typically this done by a
UART handshake output signal - (RTS?).

Isolating the RS422/485 side of the nodes is pretty essential in
industrial I/O networks. There are several ways to do this. Usually
isolated DC/DC converter and high-speed optocouplers (Hewlett Packard!).
DCDC modules are much (about ten times) dearer than 7805 regs, which
soon adds up, so some buses provide a 12-24V power rail for this
purpose. Which transfers cost to the cable. High-speed optocouplers are
not cheap either.

My recommendation: don't waste your life writing software for one-off
networks (unless someone pays you to!)

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