RS232 drivers contain an output current limiter to protect the driver against overloads due to output short circuits. They work similar to the output current limiters in OPamps: When being active, they limit the output current like a constant current source. As consequence, they limit the output voltage then, even if enough supply voltage would be available to provide enough output voltage.

Assume that this circuit



is connected to one output of SN75196 driving the RTS signal of PC. Then, following potentials could be seen internally of SN75196:






So, the output current limit, means the short circuit current is what limits the usefulness of above circuit arround TL431. But take note, the output current limit does not depend on actual potential at output. Or by other words, you need not to force it to GND potential to achieve this maximum current!
In the above circuit it's sufficient to short circuit the output to nearly 5V potential to let the maximum current flow.

The circuit arround TL431 was and is used in RS232 temperature sensors, but only needing 2...3mA supply current. This current is in the range of nearly all RS232 ports of PCs. Whether this circuit also works for Satish, who needs about 5mA is questionable: Although todays RS232-drivers deliver about 10...25mA short circuit current typically, manufacturing tolerances within the current limiting circuit can result in a some smaller short circuit current.

Kai