VK1100/VK2100 serial port connection can be divided as two parts: Regular use and port listening between VKs. 

For regular use, please refer to the VK series pin mapping that from product manual. 

For port listening between VKs, if the application is to monitor behavior between multiple VK control boxes (such as after VK1100 #1 turning on its relay1, then turn off VK1100 #2's relay2), you can utilize VK's serial interface for monitoring. 

You can either use RS232，RS422， or RS485 interfaces. 

For RS232, and RS422 , you need to use cross over cable. RS485 is pin to pin.