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D-SIM for the HP-42S

By Ton van de Burgt

Digital circuit simulator - Simulates circuits with digital components.


Input:

An N*4 matrix called: "DC" with the circuit description. Each line in the matrix contains one component.

Column 1 in the matrix indicates the type of component (1 to 7),
Column 2 indicates the output node of the component
Column 3 and 4 indicates the input nodes of the component.

Component types:

AND 1
NAND 2
OR 3
NOR 4
XOR 5
XNOR
6
Inverter
7
 

The input node of an inverter is placed in column 3.

Input Matrix DC:

1

3

7

8

2

4

1

8

3

5

2

7



Component

Output node
Input node
 Input node

Usage:

After starting D-SIM the following menu appears:



Pressing N1 to N6 set or reset the corresponding node.
Pressing [Arrow up] or [Arrow down] browse through the pages with the node numbers, of mode node numbers are available.
Set and reset the input nodes and, if necessary, give initial values to the other circuit nodes and press R/S for starting the simulation.
The number of nodes that are displayed is always a multiple of 6. The reason is an easier program.
The screen shows the line number of the input matrix that is simulated.



When the simulation is ready:

In case of a unstable circuit:

Reasons for an unstable circuit are:

1: Illegal operations with a flip-flop. See example 2.
2: a loop back in the circuit. For instance: an output of an inverter is looped back to its input.

In an Unstable situation the values of the nodes is garbage. Pressing R/S again will give different results.

 

Example 1:

1 of 4 decoder:

Input matrix ("DC"):

1

3

7

8

1

4

1

8

1

5

2

7

1

6

1

2

7

7

1

  

7

8

2

  

The truth table:

Inputs

Outputs

  

1

2

3

4

5

6

7

8

0

0

1

0

0

0

1

1

1

0

0

1

0

0

0

1

0

1

0

0

1

0

1

0

1

1

0

0

0

1

0

0

Node 1 = 0 and Node 2 = 1;

Press R/S.

Node 5 is 1; Node 7 is 1
 


Example 2:

RS Flip-Flop:


 

Input matrix ("DC"):

4

3

1

4

4

4

3

2

The truth table:

Simulation step

Inputs

Output
(Before)

Output
(After)

Remark
 

1

2

3

4

3

4

  

1

0

1

0

0

1

0

Set Q

2

0

0

1

0

1

0

Memory

3

1

0

1

0

0

1

Reset Q

4

1

1

1

0

0

0

  

5

0

0

0

0

1

1

Unstable

6

1

0

1

1

0

0

Reset Q

On such a flip flop it is not allowed to change both inputs from 1 to 0. In a real circuit one input is faster than the other one. That one will ‘win’. In a simulation both the inputs are change on the same moment. This will result in an Unstable situation.


Binary files for emulators:
Raw binary: dsim.raw  Binary for HP-42X: dsim.42x
























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