Construct a controller of the process shown in the figure Construct a controller of the process shown in the figure. The timer output (TU) is initially low when its input (TM) is low. When TM is taken high the output stays low for five minutes and then goes high. It resets to low when TM is taken low. All level sensors become true when the level is reached. The process sequence is: Fill the tank to level A (LA) from valve A (VA) Fill the tank to level B (LB) from valve B (VB) Start the timer (TM), stir (S), and heater (H) When five minutes are up take stir (S) and heater (H) off Open output valve (VC) until the tank is empty (LE) Take the timer low (TM) and go to step 1.

Solution; To provide the solution, we first form the state variable representation of the system by assignment of binary states. There are four input variables (LA, LB, LE, TU) and six output variables (VA,VB,VC, TM, S, H). A discrete state of the system is defined by specifying these variables. Because each variable is a two-state variable, we use a binary representation: true=1 and false=0. Thus, for input, if level A has not been reached, then , LA=0 and if it has been reached, then LA=1. Also, for output, if valve C is to be closed, then we take VC=0, and if it is commanded to be open, then VC=1. Let us take the binary “word” describing the state of the system to be defined by bits in the order Process outputs (Controller inputs): (LA)(LB)(LE) (TU) Process inputs (Controller outputs): (VA)(VB)(VC)(TM)(S)(H)

Initial Condition (Stand By) All outputs off start LA VA 1 LB VB LU S,TM,H LE VC

CAR 1 S 2 3 4 5 6 S0 S1 Control Outputs Control Memory (ROM) SEL 8 X 15 CAR 1 S 2 3 4 5 6 S0 S1 NXTADD1 NXTADD0 SEL Control Outputs Start

(VA)(VB)(VC)(TM)(S)(H) Control Signals • Six control signals needed. • We can use these signals as is or encode them to reduce the number of bits needed in the control word. • If we do not encode these: 6 bits needed 5 4 3 2 1 0 (VA)(VB)(VC)(TM)(S)(H) VA: 100000 VB: 010000 VC: 001000 TM: 000100 S : 000010 H : 000001

-Register Transfer Description of the Microprogram Each memory location contains a microinstruction, to be executed in the corresponding state. The register transfer statements are:

Address IniST: Start:CAR VAST, Start’ CAR İniST The above Register Transfer Operation can be translated into a symbolic microprogram (control words):   Example: Address IniST: Start:CAR VAST, Start’ CAR İniST can be written as: Address IniST:

Symbolic Microprogram: Address NXTADD1 NXTADD0 SEL CONTROL IniST VAST Start none VBST LA VA S;TM;HST LB VB VCST TU S, TM, H LE VC

Binary Microprogram