The 3504 controller is supplied in the standard ¼ DIN size (96 x 96mm front panel). The 3504 Controller contains up to six plug-in hardware modules; the 3508 Controller has up to three. Moreover, digital communications modules can be fitted in two positions.

These modules provide an interface to a wide range of plant devices and those fitted to it are identified by an ordering code printed on a label to the side of the instrument. This code defines the basic functionality of the instrument which may be:

• Controller only
• Programmer and controller
• Control type – Standard PID, valve positioner
• Digital communications type
• Options