SLC 504 DH-485 <> PC RS 232

[jjsohio]

Just finished job in customersite
...
Still, it remains for me unclear why use this DH485 instead DF1 if there is only PLC (1) and HMI (1) in the system? What's the idea?

Most likely, the HMI is a PanelView that is DH485 (such as the 2711K5A2). This requires the other device(s) to also speak DH485.
The AIC is there for 2 reasons:
1- Isolation between SLC and HMI
2- Its a network access point

You earlier comment about RS and being flexable...
They have created many different comm options for devices, it is up to the designer to select the correct comms option.

 

[Hundikoer]

The panelView was 2711K5A5 and no AIC

But yes, initial designers make the choice...