RS232-+25v  switch, Looking for the right search term - Page 1
Forum section: electronics / beginners

author: kingofkya
author_rank: Regular Contributor
timestamp: January 08, 2015, 05:25:34 pm
content: 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|0One of my though was just 2 fets with a diode in front of em or maybe a triac of some kind.

author: mrgregs
author_rank: Contributor
timestamp: January 08, 2015, 06:19:28 pm
content: The usual way is to use something like aMAX232for the level shifting.

author: Electro Fan
author_rank: Super Contributor
timestamp: January 08, 2015, 06:21:17 pm
content: Do you mean you want to "invert" the signal, or maybe something else?

author: kingofkya
author_rank: Regular Contributor
timestamp: January 08, 2015, 07:12:22 pm
content: 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.

author: Wilksey
author_rank: Super Contributor
timestamp: January 08, 2015, 07:14:02 pm
content: If it is just one RS232, use relays, depending on flow control you can just to TX and RX with a DPDT type.

author: kingofkya
author_rank: Regular Contributor
timestamp: January 08, 2015, 09:13:32 pm
content: 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

author: Paul Moir
author_rank: Frequent Contributor
timestamp: January 08, 2015, 09:16:49 pm
content: Analog switches like that one won't work. Look at Vis (analog input voltage): -0.5 <= VIS <= VCC +0.5I take it you're stuck on RS-232? RS-485 is more friendly bus wise.

author: kingofkya
author_rank: Regular Contributor
timestamp: January 08, 2015, 09:20:24 pm
content: Yeah i saw that right after i posted....

author: kingofkya
author_rank: Regular Contributor
timestamp: January 08, 2015, 09:24:02 pm
content: 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%3dI just wonder how that will effect the signal. I have a feeling it will round off the signal.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: January 08, 2015, 09:34:21 pm
content: 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?

author: kingofkya
author_rank: Regular Contributor
timestamp: January 08, 2015, 09:51:41 pm
content: 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.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: January 08, 2015, 09:59:04 pm
content: FET output opto, not SCR. Like this:http://www.digikey.ca/product-detail/en/H11F1M/H11F1M-ND/1793945Just 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

author: edavid
author_rank: Super Contributor
timestamp: January 08, 2015, 10:04:33 pm
content: 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.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: January 08, 2015, 10:07:17 pm
content: Good point!

author: kingofkya
author_rank: Regular Contributor
timestamp: January 08, 2015, 10:33:18 pm
content: 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%3dAlso 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.

author: Wilksey
author_rank: Super Contributor
timestamp: January 08, 2015, 10:34:34 pm
content: 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.

author: edavid
author_rank: Super Contributor
timestamp: January 08, 2015, 11:37:00 pm
content: Quote from: kingofkya on January 08, 2015, 10:33:18 pmNo, it's still available everywhere (although not cheap):http://www.findchips.com/search/lt1039

author: SL4P
author_rank: Super Contributor
timestamp: January 08, 2015, 11:56:15 pm
content: Depending on what's already connected to the buss, consider diodes to steer/isolate the source.Sometimes a couple of well placedschottkyscan 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.

author: rsjsouza
author_rank: Super Contributor
timestamp: January 09, 2015, 01:33:39 am
content: Quote from: edavid on January 08, 2015, 11:37:00 pmPerhaps a cheaper alternative:SN75C1406. Price atDigikey.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: January 09, 2015, 02:35:33 am
content: I don't think that one, nor the other one the OP picked out has a shut down mode.