# The truth about usb to serial converters



## betchern0t (Dec 4, 2010)

Just worked through buying a usb to serial converter. Since there are many questions up here about this thought I would share.

First thing to realise is that the serial standard (RS232C) ie how the cable is setup is very old in computer terms. This has meant that over time people have found that they can get away with things that don't match the standard. You can generally communicate with about four wires in a cable however the standard plug is 25 pins and normally 9 pins. This should tell you that careless/cheap manufacturers may not implement all pins. My 20 yr old embroidery machine (Toyota AD820) expects all 9 to work and work well. 

Secondly handshaking is important. Handshaking tells each end when to communicate a bit like the word "over" in all radio conversations you see in movies that says the person is finished talking and ready for the other end to talk. 

There are two main ways of handshaking: hardware and software or XON/XOFF. Hardware uses a series of pins in the cable by placing a voltage or removing a voltage to say to the other end whether to talk or not. If those pins aren't connected then communication doesn't happen. 

Software handshaking inserts into the conversation special words to say whether you are finished or not ie the code words: XON or transmitting (X) now (on) and XOFF or transmitting (X) ended (OFF). This is generally easier and cheaper to do so after a while it was very rare to see hardware handshaking. It tended to be much more popular in industrial settings because it was more reliable. Considering this, most usb to serial converters are designed for the home market, they're cheap so it is possible the manufacturer has saved some pennies by not implementing hardware handshaking. My machine requires hardware handshaking.

Thirdly, the voltage on usb is 5 volts. The serial standard requires voltages of +12v and -12v to signal the ones and noughts that make up the information being sent. The voltages around 0 volts are ignored as garbage. As computers developed and corners were cut, the amount of voltages around 0 volts that were ignored by various machines got less and less. This meant that over time, people could generally get serial comms to work at around -5v to +5v or less. What does this mean, many usb to serial adapters operate at -5v to +5v or less. The components to shift the voltage to 12 volts are not required and in many cases will work. However for my machine they don't. I bought three cables and one worked. 

The last thing to realise is that Windows XP is brain dead as far as usb ports are concerned. Don't know whether this is fixed in later versions. But Windows directly ties access control, device and drivers to a usb port. This will mean that if you change which usb socket you plug something into you may have problems. In my case it is linking the com port id (com5) to the usb to serial cable and usb port. If I move it the com port id has to change.

There is an in depth discussion of usb to serial cables here: USB to RS232 converters that covers the issues well.

HTH

Paul


----------



## SnapGraphics (Dec 21, 2010)

betchern0t said:


> Just worked through buying a usb to serial converter. Since there are many questions up here about this thought I would share.
> 
> First thing to realise is that the serial standard (RS232C) ie how the cable is setup is very old in computer terms. This has meant that over time people have found that they can get away with things that don't match the standard. You can generally communicate with about four wires in a cable however the standard plug is 25 pins and normally 9 pins. This should tell you that careless/cheap manufacturers may not implement all pins. My 20 yr old embroidery machine (Toyota AD820) expects all 9 to work and work well.
> 
> ...


I use a number of belkin converters and have never had a problem once I get the drivers installed. Both my Master Cutter 24" and my Roland 12" use converters.


----------



## betchern0t (Dec 4, 2010)

Yeah, Belkin is a good brand and they no doubt implement all pins at the right voltage levels. It is more hit and miss with cheaper brands but even some of the better brands will have these problems. I guess it also depends on the age of the equipment. One of the three I tried I had used in another situation perfectly.

Cheers Paul


----------



## api (Nov 22, 2009)

We use StarTech ICUSB2321X "1 Port Professional USB to Serial Adapter". 

http://us.startech.com/product/ICUSB2321X-Professional-USB-to-RS-232-Serial-Adapter 

No problems with it whatsoever. It costs $44.

Once connected, this USB to serial adapter retains assigned serial COM port values, allowing the same values to automatically be re-assigned to the port provided by the adapter in the event it is disconnected, then re-connected to the host computer. There is no need for external AC power, just install the driver and plug in the USB/Serial adapter...


----------



## jiarby (Feb 8, 2007)

A far better solution than a USB Adapter would be a PCI DB9 Serial Card.. then it is dedicated.


----------



## betchern0t (Dec 4, 2010)

Yes indeed. However when you are using laptops this may not be an option. For laptops with PCMCIA or PC-CARD slots a serial card would also be a much better option. In our case we are using netbooks with only USB ports.

Cheers Paul


----------



## PositiveDave (Dec 1, 2008)

Keyspan works for me, I tried a cheap imitation but it was flaky.


----------

