USB to RS232

Wig

I'm just wondering how a USB to RS232 (serial port) cable works?

USB has four "pins" 5 if you count the external bit which might act as a ground. And serial port has 9 pins & an external bit too. So how does the cable manage to adapt the signal?

isofa

If flow control isn't required, 2 or 3 connectors via RS-232 will work fine for transfer, only when flow-control is needed does more (5 connections I believe) become important, I doubt many devices ever used the full 9 pin connections. So I guess (I'm not certain!) that is why those cables work... (unless of course a specialist driver is used to swap the signals back and both from the transmit and receive pins etc).

espresso

Wig wrote: »

I'm just wondering how a USB to RS232 (serial port) cable works?

USB has four "pins" 5 if you count the external bit which might act as a ground. And serial port has 9 pins & an external bit too. So how does the cable manage to adapt the signal?

Basically USB has four connectors +5V D+ D- 0V The outer external part of the plug/socket is not used but a supply voltage is useful to power attached devices e.g. Flash drives etc.

RS232 has many other signals but only TX RX and Gnd are really needed to transmit and receive data.

USB is a later standard where all devices work to the same standard.

unrich

I imagine there is a small interface circuit powered by the 5v.

Nilrem

unrich wrote: »

I imagine there is a small interface circuit powered by the 5v.

yup, they aren't directly compatible so the USB to Serial connectors have a conversion chip in them (same with the usb to parallel etc connectors) .

AlwaysSomething

Yep, there's processing going on in there. I have a USB-IDE cable which lets me plug in an old hard disk drive. That's got 40 pins on the IDE end.
USB 2.0 is a pretty good standard actually. It's easy for manufacturers to write device drivers, or use existing ones, and the throughput is really fast - you can send two high def TV channels down one USB connection.

espresso

AlwaysSomething wrote: »

Yep, there's processing going on in there.............

It's not processing as such, more level changing. the processing is done by software not hardware. USB data signals are logic 0 = 0V logic 1 = 3.6V whereas RS232 uses signals that are plus or minus 3 to 15 volts i.e. they swing above and below 0V. The level conversion chip used is commonly a MAX232 details here.

USB to IDE adapters are a completely different thing.

Belnahua

We use these http://www.misco.co.uk/applications/SearchTools/item-details.asp?EdpNo=235832&CatId=760 at work, and they generate a new COM port; so we have com1 thru 4. Which is very useful when you have old legacy equipment using RS232 and new PCs with only one serial port!

Wig

Ok, that's understood now. Thanks everyone.