This booklet is the third of a series dedicated to automation recipes created with the PLC (Programmable Logic Controller) and HMI (Human Machine Interface) binomial. The series is aimed at an audience of readers with an elementary knowledge of PLC programming, eager to learn advanced solutions, extensively tested on real systems. In modern computer programming, generally oriented to the development of "object-oriented" software, the developer strives, as much as possible, to resort to so-called "Design Patterns," standard solutions for frequently recurring problems. A design pattern describes a problem, particularly recurring in a given context, and then provide the heart of the solution to this problem. It is therefore possible to successfully reuse this solution, thousands and thousands of times, with the certainty of using an efficient and well-tested solution. In the present series, which deals exclusively with development on PLC-HMI, the term "design pattern" has been replaced by the term "automation recipe" for an easier understanding by the non IT reader. In the chapters of this book we will show in detail an automation recipe that can be reused in any PLC-HMI automation project that uses machine sequencers. The recipe has also been optimized for operation with Scada supervision systems. The first book deals with electric motors automation while the second with measurement and monitoring quantities acquired with 4-20 mA current sensors. This third book deals with the topics of sequencers for starting / stopping machinery such as pumps or compressors that operate in twin or parallel mode on groups or technological plants. The sequencers process the signals output from a monitoring block of an analogue quantity and send the start command to a certain number of electric motors, which are automated with electrical blocks of type ElectricMotor (see volume 1). They therefore complete the regulation chain of an analogue quantity with digital drives (start / stop). In detail, the first section of this booklet, dedicated to the application domain, analyzes the two types of sequencer: twin for the operation of two machines, one of which is always on stand-by or parallel to start / stop a certain number of machines, generally of the same size, installed in parallel. The second section deals with the development of combined software for both PLC and HMI. The logic of the two functional blocks (UDFB), Mot2Seq and Mot6SEq, and the related display screens, fo