The interconnection of subsystems is done via simple EQUATION blocks acting as interfaces. They provide all needed inputs of the components collected within a subsystem and vice versa the outputs needed by connected subsystems or components. By using EQUATION pairs as input and output interfaces, variables can be transferred without a visual link between a pair (see also Fig. 3 in section 2.2). As a result the possibility of replacing subsystems without deleting or reconfiguring any visual connection in SIMULATION STUDIO is given. The principle is based on the fact that variables defined in equations can be transferred to components or other equations without linking them visually. Furthermore, in SIMULATION STUDIO outputs from components used within an equation must be defined as auxiliary variables on the left of the dialog window of an EQUATION block[12]. These auxiliary variables only appear in the graphical representation but they are not written to the input file of TRNSYS itself. Therefore, they can have the same names as the “real” variables (see Fig. 4). For the example in Fig. 4 the following section will be written to the TRNSYS input file:

EQUATIONS 2 "Inputs-right" input_T_in = [UNIT No., OUTPUT No.] input_mdot_in = [UNIT No., OUTPUT No.]

EQUATIONS 2 "Inputs-left"

Tin = input T in mdot in = input mdot in

In the graphical representation the expression [UNIT No., OUTPUT No.] corresponds to the auxiliary variables on the left of the EQUATION block dialog, where outputs of components can be connected to it (see also right dialog window of Fig. 4). Because of exactly same names of the variables and the auxiliary variables used for the inputs, the transfer within the EQUATION pair in SIMULATION STUDIO is not visible for the user while connecting subsystems to each other (see Fig.

5).

Fig. 5: Connection dialogs of EQUATION pair in SIMULATION STUDIO. The user does not realize the
transfer of variables within the EQUATION pair while connecting subsystems or components because of the
same names of input (auxiliary) variables (right dialog window) and output variables (left dialog window) in
SIMULATION STUDIO. In this example the temperature and mass flow rate of a store is transferred to a pump,

which is located within a subsystem.