Author Topic: RS232-+25v switch, Looking for the right search term (Read 6116 times)

kingofkya · « **on:** January 08, 2015, 05:25:34 pm »

Basicly I just want to switch an rs232 signal on to and off of a bus.

So I can find plenty of devises that will switch that works with positive voltage, but rs232 uses negative 25v to positive 25(most stuff is lower than 25 though). I have had a few though on how to get around it but i want to know if there is a part that will do what I want so.

I was looking in here but i cant seam to find anything.
http://www.mouser.com/Semiconductors/Switch-ICs/Analog-Switch-ICs/_/N-7590c/?Ns=Pricing|0

One of my though was just 2 fets with a diode in front of em or maybe a triac of some kind.

mrgregs · « **Reply #1 on:** January 08, 2015, 06:19:28 pm »

The usual way is to use something like a MAX232 for the level shifting.

Electro Fan · « **Reply #2 on:** January 08, 2015, 06:21:17 pm »

Do you mean you want to "invert" the signal, or maybe something else?

kingofkya · « **Reply #3 on:** January 08, 2015, 07:12:22 pm »

Well if possible i don't want to convert to ttl just deal with the signal as is. So rs232 on the bus and the ability to switch a device on and off the bus.

Wilksey · « **Reply #4 on:** January 08, 2015, 07:14:02 pm »

If it is just one RS232, use relays, depending on flow control you can just to TX and RX with a DPDT type.

kingofkya · « **Reply #5 on:** January 08, 2015, 09:13:32 pm »

I was goan say i want to keep it small but i had no idea you could get relays like this.

http://www.mouser.com/ProductDetail/ON-Semiconductor/NLAST4599DFT2G/?qs=sGAEpiMZZMtqO%252bWUGLBzeBJ%252biJ%252bAlmpi

Paul Moir · « **Reply #6 on:** January 08, 2015, 09:16:49 pm »

Analog switches like that one won't work. Look at Vis (analog input voltage): -0.5 <= VIS <= VCC +0.5

I take it you're stuck on RS-232? RS-485 is more friendly bus wise.

kingofkya · « **Reply #7 on:** January 08, 2015, 09:20:24 pm »

Yeah i saw that right after i posted....

kingofkya · « **Reply #8 on:** January 08, 2015, 09:24:02 pm »

I am thinking, I should try to just throw a scr/triac across it. http://www.mouser.com/ProductDetail/Lite-On/MOC3022S-TA1/?qs=sGAEpiMZZMteimceiIVCB4wfliURD97K%2fE0dMo%252bBKxo%3d

I just wonder how that will effect the signal. I have a feeling it will round off the signal.

Paul Moir · « **Reply #9 on:** January 08, 2015, 09:34:21 pm »

You'd need to drive the gate both ways so it would be two SCRs, one of which would need a negative voltage. Then you'd have to figure out how to turn it off. Possibly a FET optoisolator might work easily enough?

kingofkya · « **Reply #10 on:** January 08, 2015, 09:51:41 pm »

Yeah thats what i am thinking the opto would make it easy to drive. Just need to test it to see if it cases any data issues. I think it might roud the edges due to the scr turning on.

Paul Moir · « **Reply #11 on:** January 08, 2015, 09:59:04 pm »

FET output opto, not SCR. Like this:
http://www.digikey.ca/product-detail/en/H11F1M/H11F1M-ND/1793945

Just makes it on breakdown voltage (+-15v RS-232 not 25). You'd probably want to drive it at 10mA each to get the resistance down nice and low.

EDIT: Toshiba's got some higher voltage ones: TLP3119

edavid · « **Reply #12 on:** January 08, 2015, 10:04:33 pm »

RS232 and buses don't really go together - what is it you are trying to do?

Anyway, since it's not a bidirectional signal, you don't need a switch. Just use an RS232 transceiver with shutdown, for example the LT1039.

Paul Moir · « **Reply #13 on:** January 08, 2015, 10:07:17 pm »

Good point!

kingofkya · « **Reply #14 on:** January 08, 2015, 10:33:18 pm »

Yeah perfect i knew there was a simple solution that part you linked is eol.

Buy anyways i am sure there are others looking now.
Could probably make this work too dosen't have the shutdown but has enfo i/o that i could make it work.
http://www.mouser.com/ProductDetail/Texas-Instruments/GD75232PWR/?qs=sGAEpiMZZMutXGli8Ay4kCi9xQZvT4qAT8kgJZ1ZuVc%3d

Also its not really a bus but thats just an easy way to describe it. Basically i am making a serial console switch so i can switch between devices only one will be active on the "bus" at a time and the receiving end obviously.

Wilksey · « **Reply #15 on:** January 08, 2015, 10:34:34 pm »

I was thinking relay rather than analog switch, I have used some quite small 5v relays from Farnell /Element 14 in the past for similar things.

But as others have said, RS232 is not usually good on buses.

edavid · « **Reply #16 on:** January 08, 2015, 11:37:00 pm »

Quote from: kingofkya on January 08, 2015, 10:33:18 pm

Yeah perfect i knew there was a simple solution that part you linked is eol.

No, it's still available everywhere (although not cheap): http://www.findchips.com/search/lt1039

SL4P · « **Reply #17 on:** January 08, 2015, 11:56:15 pm »

Depending on what's already connected to the buss, consider diodes to steer/isolate the source.

Sometimes a couple of well placed schottkys can provide a simple 'no-moving-parts' merge of true +/- RS232 streams.
Also works with singler-ended signals as well - just work it through on paper.

rsjsouza · « **Reply #18 on:** January 09, 2015, 01:33:39 am »

Quote from: edavid on January 08, 2015, 11:37:00 pm

No, it's still available everywhere (although not cheap): http://www.findchips.com/search/lt1039

Perhaps a cheaper alternative: SN75C1406. Price at Digikey.

Paul Moir · « **Reply #19 on:** January 09, 2015, 02:35:33 am »

I don't think that one, nor the other one the OP picked out has a shut down mode.


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Author Topic: RS232-+25v switch, Looking for the right search term (Read 6116 times)

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