Some PCs serial ports can have levels of -15 volts and +15 volts (while still adhering to the RS232 specification). However, the E3 input circuitry may be designed for levels of 0 volts and 5 volts. If you're feeling protective of your E3, then using the following circuit may put your mind at rest. It works fine for my E3 and mind.
PC E3
9-pin 3-way
D-connector jack plug
100R
(Tx) 3 o------/\/\/\----+-------------+-------------o Ring
resistor | |
| \ |
--- ---
^ IN4148 ^ \ BZY88 4v7
/ \ diode / \ zener diode
--- ---
| |
| |
(0V) 5 o----------------+-------------+-------------o Sleeve
(Rx) 2 o--------------------------------------------o Tip