usb - serial adapters that do 12V /  all pins - Page 1
Forum section: electronics / general

author: rr100
author_rank: Frequent Contributor
timestamp: January 26, 2013, 09:48:26 pm
content: I'm looking for acheapUSB-serial adapter that would actually do "close to real" "PC-serial" on the other side, most of them would skimp and either do 5V or do some other nasty things. Obviously ebay is full of them but no way to really tell what's good, unless you go to tens of dollars (or actually euros in my case). I'm sure there are some for a few bucks including shipping from China but which ones?I know I can easily make my own but it'll probably cost more and I'm hopeless at making/improvising cases - it'll probably end up as a mass of duct tape.

author: 8086
author_rank: Super Contributor
timestamp: January 26, 2013, 10:54:50 pm
content: You can make one pretty easily with a FT232R and MAX3243E.It wouldn't necessarily cost more.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 26, 2013, 11:06:12 pm
content: Why do you need 'full' 12V? 0-5V works fine in almost all situations.

author: Simon
author_rank: Unknown
timestamp: January 26, 2013, 11:59:43 pm
content: The USB specification is 5V, so no adapter will do 12V unless you add the levels transitioning.

author: Unknown
author_rank: Unknown
timestamp: January 27, 2013, 12:39:16 am
content: Many USB-to-RS232 converters will incorporate a charge pump to produce larger positive and negative voltages. For example the well known MAX232. I'm quite sure that RS-232 requires negative voltages, so no way to directly drive it from just a 0 to 5 V supply anyway.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 27, 2013, 12:56:31 am
content: Quote from: alm on January 27, 2013, 12:39:16 amMany cheap USB-232 boxes use 0V/+5V levels. This usually talks just fine to standard RS232 ports.

author: rr100
author_rank: Frequent Contributor
timestamp: January 27, 2013, 08:23:20 am
content: I'm not talking to standard RS232 ports, as in connecting two PCs over a null-modem cable, I'm talking to all kinds of weird adapters, usually powered from serial (simplest example would be the usual serial-optocoupler multimeter-PC interface). Some "just work", some don't. One I reverse engineered and I'm powering it from a differential power supply and I can tweak it to output any levels I want, be it for PC or 3-5V.But I want something that obeys the standard and therefore would have more chances to "just work" all the time. I don't fancy paying 40-70+ (!!!) EUR for such adapter (I can buy multiple RS232 capable computers for that money!). It's really easy to make but I'd rather spend my time doing something else (and as I said the way I deal with cases and such it would end up being a big blob of duct tape/electrical tape).USB-serial adapters start on ebay from 1.x$ shipped! The cheapest are most likely the "5V" variety, question is: anybody knows some that aren't? Maybe for 5$ or for 10$?

author: Monkeh
author_rank: Super Contributor
timestamp: January 27, 2013, 08:49:08 am
content: Quote from: mikeselectricstuff on January 27, 2013, 12:56:31 amI invite you to try talking to my Sun boxes or any of my switches with one of those!

author: SeanB
author_rank: Super Contributor
timestamp: January 27, 2013, 09:19:14 am
content: If you need the true +-12V levels then just get the cheap ones and add 2 1488/1489's to make the null modem connection. You will need a +-12v supply for them though, and probably this will need powering from an external source if you want a full power link, as the power needed is more than USB can supply, you need +-12V at 500ma per rail to drive the link. Thus the power supply will have to give 5V along with +-12V supplies.

author: codeboy2k
author_rank: Super Contributor
timestamp: January 27, 2013, 09:49:54 am
content: I use a Keyspan model USA-19HS. It wasn't $5-$10 cheap, but it also wasn't really expensive either. I've had it a few years now.It does +- 6V, not 12V but it seems to work with everything I've ever tested it with.That includes a few old terminals and modems from the 80s, some Cisco gear from the 90s.I think the older gear requires the negative voltage swings and anything that only does 0-5V positive swings won't work with the older gear or anything that adheres to the +-12V standard. But I think the thresholds are low enough that a -5 to -6V swing is sufficient .You can find one on ebay, but it's gonna set you back about 30 Euros or so.It's a DB-9 device, but it fully implements the +/- 6V on all pins and does CTS/RTS DSR/DTR RI CD TXD RXDHere's a scope capture showing +5.47V / -5.78V output on the TXD pin, when pressing the ENTER key on my PC:

author: Monkeh
author_rank: Super Contributor
timestamp: January 27, 2013, 09:52:26 am
content: Except for picky devices, swinging past ±3V should work. ±5V or so works with most anything. 0V is totally undefined.

author: rr100
author_rank: Frequent Contributor
timestamp: January 27, 2013, 10:22:45 am
content: You don't need to pay 30 euros for a 5V (or 5.x, I think that depends on your USB voltage) one, the 1.x$ ebay would probably do.Yes, I know many/most self powered "network" devices like routers, servers, terminal, modems would "just work". I don't care about those, and when I care I have a MAX232CPE adapter that would do 12V levels (but just one TX and one RX) so no problem here.Anyway I think we're going in the wrong direction here: I'm not choosing the device to connect here (like a router or server or whatever), I know if I chose now something I'll chose something with USB or bluetooth or whatever. What works nicely works, but I'm interested in an adapter that would obey the standardfor the devices I already have and don't work.

author: Monkeh
author_rank: Super Contributor
timestamp: January 27, 2013, 10:25:29 am
content: You've answered your own question: A charge-pump driver to reach appropriate levels. If you want something ready made:http://www.easysync-ltd.com/product/647/usb2-f-1001-x10.html

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 27, 2013, 10:33:47 am
content: If you look at the datasheets of probably the two most common RS232 receivers - 1489 and max232, both have input thresholds above 0V, and so will work with 0v/5v levels.Agreed, anything that tries powering from RS232 could well have problems, however for simple communications, 0V/5V will work in the vast majority of cases.Many years ago I sold a couple of dongle type products that powered from RS232 (about 1mA) and used 5V levels - out of maybe 500 units sold never had a single issue with compatibility.QuoteAlthough 0V is undefined in the standard, it is well defined in receiver datasheets, as manufacturers know that people want the line to be idle when nothing is connected. RS232 input lines are pulled low to 0V, not V-A 9 pin D connector is also strictly speaking non-standard!Any 'picky' devices are likely to be so because they themselves do not conform to standard.If you need to be able to talk to any random RS232 device then it make sense to use a proper transceiver, however in many cases you know what your device will be talking to, and it may well be an unnecessary use of cost and power.I have no problem with USB-232s's being 0V/5VAs long asthis is clearly indicated in the spec (which of course it usually isn't, except indirectly in the price).When buying cheapo stuff from China, RS232 just one of many standards it probably doesn't meet....