This means that, to store the same size control program, the system in Figure 5-5a would require 8K memory capacity instead of 4K, as in Figure 5-5b.Īfter becoming familiar with how memory is utilized in a particular control- ler, users can begin to determine the maximum memory requirements for an application. With these memory requirements, the effective storage area of the memory system in Figure 5-5a is half that of Figure 5-5b. Suppose that each normally open and normally closed contact instruction requires 16 bits of storage area. To illustrate memory capacity, let’s refer to Figure 5-5. The manufacturer can supply this data if the product literature does not provide it. The term memory utilization refers to the amount of data that can be stored in one location or, more specifically, to the number of memory locations required to store each type of instruction. S ECTION Components The Memory System C HAPTER 2 and SystemsĪdditional information concerning how program instructions are stored will help to make a better decision. Knowing this maximum number alone is not enough to determine memory requirements. The memory capacity of a programmable controller in units of K is only an indication of the total number of storage locations available. Block illustration of (a) a 4K by 8 bits storage location and (b) a 4K by Figure 5-5 illustrates two memory arrays of 4K each however, they have different configurations-the first configuration uses one-byte words (8 bits) and the other uses two-byte words (16 bits).įigure 5-5. A 1K memory, then, contains 1024 storage locations, a 2K memory contains 2048 locations, a 4K memory contains 4096 locations, and so on. Larger controllers allow memory expandability, since the scope of their applications and the number of their I/O devices have less definition.Īpplication memory size is specified in terms of K units, where each K unit represents 1024 word locations. Small PLCs have a fixed amount of memory because the available memory is usually more than enough to provide program storage for small applications. Memory capacity is nonexpandable in small controllers (less than 64 I/O capacity) and expandable in larger PLCs. Knowing memory capacity requirements ahead of time also helps avoid the purchase of a controller that does not have adequate capacity or that is not expandable. Specifying the right amount of memory can save the costs of hardware and time associated with adding additional memory capacity later. Memory capacity is a vital concern when considering a PLC application. M E M O RY C A PA C I T Y AND U T I L I Z AT I O N