Octagon 5066 with DOS 6.22, 485 modbus rtu driver

Hello. Could you help me sketch a plan of action.

I have an old mini computer Octagon 5066 with DOS 6.22 OS system. The board has 485 interface. There is also another device which transmitting parameters using 485 Modbus RTU. So I need to develop a program to poll this device.

I only have some skills of programming Atmega controller in C and programming on Delphi.

So roughly speaking, is the Octagon 5066 like odinary controller and the approach to writing drivers is the same? Should I use the operating system DOS to get access to 485 port? As I understand it only works with the file system? Should I work with the registers of port 485 like in a odinary controller? There is no information about 485 registers in dataseet on Octagon 5066.

Whitch language should I use to write driver C, C ++? Do I need an assembler?

I do not understand the structure. As I understand I will have an Autoexec.bat file consisting link to my driver. This driver periodically polls my sensor and in a certain place of memory is the value I need. And my main program has access to this area of ​​memory. Right?

What an easy and modern program can I develop in order to master this project? The more documentation and examples the better. Maybe it will be a DOS emulator on the computer and exchanging on rs232 with the controller?

What do I need to know? Thank you in advance.