# voltage difference on RS232 in pc and laptop

Joined Nov 14, 2014
56
hi
i have a usb to RS232 converter for connect to my laptop . When I measured the voltage on tx and rx , Voltage is approximately 6 volt(in idle time) . but voltage on this points on computer case is approximately 10 volt (in idle time) .
What is the reason for this difference ؟؟

Joined Jul 18, 2013
22,038
Are you talking the voltage of 6v on the convertor?
The standard originaly was +9v to -9v now you may find anything from 5v to 15v in practice. The important part is the + & - swing.
Max.

Joined Nov 14, 2014
56
yes voltage is 6 volt on converter
My question is why is 10 volt (in idle time) on all computer case or 6v on converter or on some PLCs such as fatek plc is 12 volt or ....
This voltage depends to what cases ??

Joined Jul 18, 2013
22,038
It all depends on the method and/or IC that is used for the RS232 transmission, for example the popular MAX232 produces a +7v & -7v from a charge pump arrangement using the 5vdc supply.
As long as the receiving end is capable of interpreting the signals it should be ok if you do see a difference between one end of a system to another.
Max.

#### ian field

Joined Oct 27, 2012
6,539
hi
i have a usb to RS232 converter for connect to my laptop . When I measured the voltage on tx and rx , Voltage is approximately 6 volt(in idle time) . but voltage on this points on computer case is approximately 10 volt (in idle time) .
What is the reason for this difference ؟؟
The official standard for RS232 is +/- 12V, but chips like the MAX232 TTL to RS232 converters only double the available 5V Vcc.

With a desktop/tower PC, you should have both 12V rails from the PSU box - but they haven't included RS232 for some time, so I wouldn't be surprised if they've already dropped the -12V rail.

There's rarely convenient +/-12V rails on laptops.

Joined Jul 18, 2013
22,038
RS232 is just about the oldest method of computer communication, way before the implementation of the internet, people used to communicate data via a BBS (Bulletin Board) system over phone lines with a RS232 Modem hooked up to their PC.
I do a lot of work in the field with machinery that still uses RS232 and my three lap tops I use are around +-8v to +-9v on the DB9 port.
The latest published signal threshold voltages I have for RS232 Mark/Space is +3v +25v and -3 -25VDC.
Max.

#### ian field

Joined Oct 27, 2012
6,539
RS232 is just about the oldest method of computer communication, way before the implementation of the internet, people used to communicate data via a BBS (Bulletin Board) system over phone lines with a RS232 Modem hooked up to their PC.
I do a lot of work in the field with machinery that still uses RS232 and my three lap tops I use are around +-8v to +-9v on the DB9 port.
The latest published signal threshold voltages I have for RS232 Mark/Space is +3v +25v and -3 -25VDC.
Max.
The voltage ranges you mentioned seem to ring a bell - but I didn't say anything before because I wasn't certain.

The most recent MOBO I bought has no LPT at all, and there's only a single RS232 header somewhere on the board - not a 9-D connector on the back edge.

I vaguely remember seeing a PSU box with no -12V rail. the -5V rail fell by the wayside some time ago.