A HC4538 is a dual monostable multivibrator. How is this used in relation to your RS485 conversion? As a fail-safe timeout to avoid continuous transmit mode?

You really have to supply a schematics or more information about your circuit for anyone to be able to help.

On the RS485 side you would normally have termination of signals to reduce problems with reflections.

But when you have a RS232-RS485 converter that you disconnects on the RS232 side, you have many other issues. Where does it get its power from? Will it still be powered after the RS232 cable is disconnected? Some converters gets the power from the PC port. Next thing is what input circuitry they have - how they consider what is a low or high input signal. Will the HC4538 be powered, but have floating inputs? Floating inputs are never good. Unless a chip have internal pull-up/pull-down a floating input can function as an antenna, picking up lots of noise. And it can also result in a very much higher current consumption in the chip, which can affect other logic.