Attached is a circuit of a Crouzet millenium 3 programming cable,Am correcting in the fact that it uses the negative part of the rs232 signal which is not present in most usb to serial converters

 

RS232 requires a voltage lower than -3V to indicate a logic 1.

 

RS232 requires a voltage lower than -3V to indicate a logic 1.

But most usb to serial converters only supply positive signals!

 

But most usb to serial converters only supply positive signals!

USB to serial converters do, but USB to RS232 converters must output >+5V and <-5V (and the logic levels are inverted, MARK is negative and SPACE is positive)
If they contain a MAX232 they will output ±7V

 

Thanks for the explanation,I now know why a serial to usb converter will not work with this cable.Hopefully the rs232 express card I have will work.

 

The USB to serial converters have TTL level outputs and connect directly to the output pin of a microcontroller.
Unfortunately, there's nothing to say that some manufacturers haven't used the terms Serial and RS232 interchangeably.