The reason is better usage of the RAM memory, dedicated for FB. In standard Vision models – V120/230/260/280/290/530 FB RAM memory is 8Kbytes. One FB with Auto-tune takes 448 bytes, while the lighter FB PID – only 256 bytes. Having two FB PID enables the user to manage better the memory resources of his project. In enhanced models V130/350 the memory is 16Kbytes and in V570 – 32Kbytes. Having seriously increased amount of memory eliminates the need to handle two different FB PID. Important notes: PID algorithm in both FB is exactly the same! Auto-tune is not a “must”. It’s an independent block, which helps the user to tune easily and faster his PID loop. If the programmer will not add the module “Run Auto-tune” to his Ladder he will have the “standard” FB PID.