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Digital RS232 Switch

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2007-2009 » Archive through 14 May, 2009 » Digital RS232 Switch « Previous Next »

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amr_bekhit
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Username: amr_bekhit

Post Number: 417
Registered: 06-2005


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Posted on Wednesday, 13 May, 2009 - 10:32 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hello all,

I'm looking for something similar to an RS232 KVM switch with 2 inputs and a common but the switch can be controlled digitally using an electronic circuit rather than using a mehanical switch. Alternatively, does anyone know of an IC which I can use to do the same job?

--Amr
Helm PCB - My personal site.
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joe
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Post Number: 540
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Posted on Wednesday, 13 May, 2009 - 10:47 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Depending on how many signals you want to switch, how about a 3 or 4 pole relay and a transistor to control the switching ?

Joe
Do one thing each day that scares you – work here !
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amr_bekhit
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Post Number: 418
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Posted on Wednesday, 13 May, 2009 - 10:51 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi Joe,

I just need to be able to switch the TX and RX signals between two devices into a common port.

--Amr
Helm PCB - My personal site.
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joe
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Posted on Wednesday, 13 May, 2009 - 12:14 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

In which case, you could use a standard 2 pole change over relay.

Might be the simplest option - and not much to go wrong really.

Joe
Do one thing each day that scares you – work here !
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vlf
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Posted on Wednesday, 13 May, 2009 - 12:20 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi, thought of using Cmos gate switch; see URL:

http://www.kpsec.freeuk.com/components/cmos.htm#4001

Regards, Brian.
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joe
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Posted on Wednesday, 13 May, 2009 - 12:40 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

True RS232 is +/- 12v so you can't throw that through standard CMOS gates. You also need to think about voltage spikes and what happens if theres an accident and you get something unexpected down your cable. Standard CMOS gates arn't upto the task.

You could look at Maxim's range of analogue switches: http://www.maxim-ic.com/products/switches/80v/

They may be suitable.


Or you could make a switch using a couple of MAX232's and some basic logic to handle the TTL side signal routing. If you want more info on this drop me a line.

However, assuming thre are no other constraints and only needing two ports, don't think you can get simler than a relay.

Joe
Do one thing each day that scares you – work here !
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bignev
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Posted on Wednesday, 13 May, 2009 - 01:02 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

You could use a 74HC157 or similar multiplexer - put a series resistor on each input, say 3K3 or above. The input diodes on the HC157 would clamp the voltage to 0V6/5V6. The 0 to 5V outputs will still drive an RS232 input ok, but you might want to put say, a 470R resistor on the outputs for protection.
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joe
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Posted on Wednesday, 13 May, 2009 - 01:22 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Quote "The 0 to 5V outputs will still drive an RS232 input ok"

Depends on what device is being driven as to if 5v will work or not. Most modern equipment that I've come across won't compain, but I've got a workshop full of older "stuff" that most certainly won't work.

You also get limited noise immunity at 5v.

So, depends on the application.

Joe
Do one thing each day that scares you – work here !
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amr_bekhit
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Post Number: 419
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Posted on Wednesday, 13 May, 2009 - 02:40 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi all,

Thanks for your suggestions. I've opted to go with a DPDT signal relay - should work just fine.

Thanks for your advice!

--Amr
Helm PCB - My personal site.

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