A/C service ports - which is which?

On a 1982 300D, which is the high side and which is the low side service port for charging the A/C? There is one port that goes across the front of the engine on top on a hose just behind the fan and there is one underneath on the passengers side of the engine.

[chowpit]

The one that goes across the front of the engine is the low side (suction).

Thanks, that is what I suspected but the problem is, I just installed a new compressor, and when I try to put freon into the system using that port, I get a lot of back pressure. There is no suction at all. Any ideas?