By Miles Nicholson
“A block diagram is a diagram of a system in which the principal parts or functions are represented by blocks connected by lines that show the relationships of the blocks. They are heavily used in engineering in hardware design, electronic design, software design and process flow diagrams.
Block diagrams are typically used for higher level, less detailed descriptions that are intended to clarify overall concepts without concern for the details of implementation. Contrast this with the schematic diagrams and layout diagrams used in electrical engineering, which show the implementation details of electrical components and physical construction.” 
AutoCAD Electrical is supplied with libraries for detailed schematics, single line diagrams and footprints. You may wish to create your own category for a 4th representation in the form of a block diagram.
The block diagram could even include manufacturers images of the components giving the diagram a greater visual impact. Images can be inserted using the Insert > OLE Object (INSERTOBJ) and its recommended to embed rather than link the image.
A symbol could typically look like:
In this symbol preview, you will note the attribute WDTYPE which would have a value of “BD” or “1-“ to differentiate between a schematic symbol. Defining a value in the WDTYPE attribute ensures the part is not listed twice in the reports such as a bill of materials.
The symbol graphical representation could also be in the form of a scaled down footprint style so the style is entirely a user’s preference:
Cable connections can be created by inserting wire connections from the Symbol Builder Attribute Editor thus meaning that process lines or cables can be drawn between equipment.
For PLC’s, each slot could be represented individually meaning that the block diagram has a linked image of each slot. The methodology to link detailed block diagrams symbols to the detailed schematic is as per single line diagram association.
The final symbol in the block diagram looking similar to: