You have an Arduino Mega2560 with four hardware serial ports, so I do not see any reason that you need to use SoftwareSerial, so get rid of it in all cases. You do not appear to have any code that uses it anyhow.
I looked at the company which produce the AMAP99 series boards, in Czech Republic and for reference this is a link to the enhanced model which has Ethernet: AMiT Automation
You do not say what communications module is in the AMAP99, as it states on the web site that communications modules need to be ordered separately ?
Here is a link I think you will find useful, put together by one of our forum mods down here in Australia, Nick Gammon. It details a lot of good information on formerly RS-485, which is now TIA-485: Gammon Forum : Electronics : Microprocessors : RS485 communications
In the document above, you will learn that you will need time for the sending of data out through the serial port before you can take the pin that controls Tx/Rx direction low from transmitting to receiving.
Anyhow, I'll be interested to hear how you go.
Once you understand the AMAP99 data protocol, you will then be able to implement the correct communications code into the Arduino.
Paul - VK7KPA