How do you access objects created in app designer in a separate class script?

I've created an serial object to talk to piece of equipement in app designer using a seperate serial class script.

app.XYComm = SerialConnect(port,115200,8,1,'none','none');

i would like to read and write to this com port via a seperate class script where I have all of the functions whioch control the behaviour of this piece of equipment.

anyone got any tips on how best do this