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RS232 - RS485 - RS232 adapter

 
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Einly



Joined: 10 Sep 2003
Posts: 60

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RS232 - RS485 - RS232 adapter
PostPosted: Fri Aug 20, 2004 8:35 pm     Reply with quote
I want to use PC communicate with PIC in a long distance communication, and thus I need

1 RS232 to RS485 adapter

1 RS485 to RS232 adapter

Anybody has the reference circuit or project?

Pls help, thanks...
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Einly
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PostPosted: Sat Aug 21, 2004 12:05 am     Reply with quote
SN75176 is a 485 drivers



kruskal
Ttelmah
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Re: RS232 - RS485 - RS232 adapter
PostPosted: Sat Aug 21, 2004 2:25 am     Reply with quote
Einly wrote:
I want to use PC communicate with PIC in a long distance communication, and thus I need

1 RS232 to RS485 adapter

1 RS485 to RS232 adapter

Anybody has the reference circuit or project?

Pls help, thanks...

There are quite a few people selling such units. They are not quite as 'simple', as may at first appear. The 'key' is that RS485, is normally implemented using just one pair of wires, as a 'half duplex' link. So the drivers, have to sit in 'receive' mode, and when any data arrives on the incoming RS232 transmit line, something has to operate the signal to reverse this driver, and start sending the data across the 485 link. Also, you should look carefully at the voltages on each unit, since it could well be worth considering opto-isolating the drive. A 'minimum' circuit to do all this, then has a RS232-5v logic driver chip (I use a DS275, which is small, and adequate for the short distances that the RS232 signal has to travel), then a 5v to 5v isolated power module, and two opto couplers, one in the RX line, and one in the TX. Then on the 'isolated' section of board, there is a small PIC (12508), running a program that when it sees data arrive on the TX line, operates the direction control line on the RS485 driver, and holds it operated for 10.5 bit times (using 8 bit even parity, one stop). The driver is a simple MAX485. Remember then that the RS485 lines require termination at each end, and you also either have to provide a small amount of voltage bias to the line (so it idles 'off'), or use a driver that is designed to do this.
I can send you a .pdf, of a typical circuit for this.
A simpler version, doesn't have the opto-couplers, and 5v inverter, but doesn't then give the nice isolation.
I remember a couple of years ago, a company using this type of link, contacted me, because it was being slightly unreliable. Went to look at the link, and found that one of the wires was in contact with a 440v supply line!... The link was still running, but the error rate had risen, when units were switching on the power line. Got them to repair their wiring, and it all ran happily again.

Best Wishes
dyeatman



Joined: 06 Sep 2003
Posts: 1912
Location: Norman, OK

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RS485 info
PostPosted: Sat Aug 21, 2004 9:51 am     Reply with quote
Here is a great site with lots of RS232/RS485 info.

http://www.bb-elec.com/tech_articles/rs422_485_app_note/table_of_contents.asp

If you use full duplex (4 wires) rather than half duplx (2 wires) the hookup is very simple and requires just the drivers and bias resistors.

Here is another good one:

http://www.rs485.com/
mkent



Joined: 09 Sep 2003
Posts: 37
Location: TN, USA

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PostPosted: Sat Aug 21, 2004 11:52 am     Reply with quote
Hi Einly, If you just want a single channel link you should use RS422. RRS485 is a multiple channel protocall. Try http://www.bb-elec.com/serial_long_distance/long_distance_connections.asp
Mick
treitmey



Joined: 23 Jan 2004
Posts: 1094
Location: Appleton,WI USA

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PostPosted: Mon Aug 23, 2004 9:16 am     Reply with quote
Also something to consider is IF the wire runs outside or building to building. Then REMEMBER to account for lightning protection.
I use a opto isolated MAX1480 BCPI and an TVS 712 from semtec.
treitmey



Joined: 23 Jan 2004
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Location: Appleton,WI USA

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PostPosted: Mon Aug 23, 2004 9:21 am     Reply with quote
sory,.. that should read semtech sm712.
http://www.semtech.com/products/products_line_moreinfo.asp?ProductID=232
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