RS-422's Tx+ and Tx- mapped to RS-232's Rx and Gnd... issues?

[spaznrq]

I have a system that outputs serial data through Tx+ and Tx- pins through RS-422 format. Supposedly, these pins swing betwen 0V min to 6V max as a differential output. Instead of purchasing new RS-422 to USB cables, I already have some RS-232 to USB cables I want to use instead. Would there be any issues I can't think of if I connect the Tx+ and Tx- output pins directly to the RS-232's Rx and Gnd pins?

 

[Noway2]

See this wikipedia article, specifically the part on voltage levels:

http://en.wikipedia.org/wiki/RS-232.

The formats are not electrically compatible and I highly doubt this would work. Typically, I have see the differential between the signals to be as little as .2V. While I have converted between RS-422/485 and RS-232 many times, including using USB converters, I have never done what you are describing. I would recommend getting a converter.

 

[DRWeig]

Your RS232 to USB cables are not just wires, there is a converter in there.

RS232 voltages are NOT differential, they are referenced to ground. The voltages are ±15V nominal, though some operate as low as ±6V. Nominal RS422 voltages are ±2V.

Go here for a comparison, look at the chart near the bottom:

http://www.rs485.com/rs485spec.html

Then shop for RS422 to USB converter. I suggest newegg.com, great prices and great service.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[VEBill]

Such USB to RS422 adapter cables are also available on many of the Hong Kong online vendors, for EXAMPLE. $25 price class. Confirm details to ensure applicability to your application.

 

[mikekilroy]

if u tie T- to gnd that is a short on your output signal and could blow the rs422 output driver, depending on design Obviously not a good idea.

If you are SURE the voltage is only 0-5/6v, AND your cable length is less than around 10' (reason for differential vs rs232 - noise), then you probably can do as we have done in the past:

RS422 RS232
------------------------
T+ (or T-) -> R
R+ (or R-) -> T
COM -> COM

We have done this many times in the past to test signals and read the rs422 data on a rs232 only computer in a pinch.....

 

[spaznrq]

@mikekilroy So you would leave either T+ or T- from the output device disconnected/floating? My cable length will definitely be shorter than 10ft so your suggestion might actually work for me. Thanks!

 

[mikekilroy]

yes, ignore the other T&R line.