# How to implement discrete dynamic system in Xcos v2024.1.0? **URL:** \ **Category:** Xcos\ **Created:** [January 27, 2026, 4:10pm UTC](https://scilab.discourse.group/t/how-to-implement-discrete-dynamic-system-in-xcos-v2024-1-0/1114 "2026-01-27T16:10:19Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![Student](https://avatars.discourse-cdn.com/v4/letter/s/ecc23a/32.png) [@Student](https://scilab.discourse.group/u/Student)\ **Post date:** [January 27, 2026, 4:10pm UTC](https://scilab.discourse.group/t/how-to-implement-discrete-dynamic-system-in-xcos-v2024-1-0/1114/1 "2026-01-27T16:10:19Z")
I have been developing a Xcos simulation where I have a disturbance observer in discrete domain. My intention was to implement the observer itself as a Scilab function invoked in periodic manner from the _scifunc\_block\_m_ block i.e. the block has an event port connected to the clock source. May I ask you for an advice how to properly handle the observer state. In other words I need to have some variables which values are persistent between the individual clock events. I have noticed that the _scifunc\_block\_m_ has some discrete state in the configuration dialog. Is it intended for such usage? If so, how can I access to the state in the function code? Thanks in advance for your answer. ---
**Author:** ![Student](https://avatars.discourse-cdn.com/v4/letter/s/ecc23a/32.png) [@Student](https://scilab.discourse.group/u/Student)\ **Post date:** [February 10, 2026, 3:04pm UTC](https://scilab.discourse.group/t/how-to-implement-discrete-dynamic-system-in-xcos-v2024-1-0/1114/2 "2026-02-10T15:04:53Z")
I have decided to implement much more simple discrete system at the beginnig. I have attempted to implement the discrete first order low pass filter. So I have just defined the below given function in a separate _.sci_ file ```auto function [xnew, y1]=Digital_low_pass_filter(flag, x, u1, rpar, ipar) disp("ARGS: " + string(argn(2))); disp("FLAG: " + string(flag)); select flag case 1 then // system output update (occurs in every time step of the simulation) disp("Flag value: 1"); y1 = x; xnew = x; case 2 then // system dicrete state update (occurs only at discrete event) disp("Flag value: 2"); T = rpar(1); tau = rpar(2); a = T/(T + tau); b = 1 - a; xnew = a*u1 + b*x; y1 = []; case 3 then // continuous state update y1 = []; xnew = x; case 4 then // init disp("Flag value: 4"); xnew = 0; y1 = []; case 5 then // end of simulation disp("Flag value: 5"); y1 = []; xnew = x; case 6 then // derivative computation disp("Flag value: 6"); y1 = []; xnew = x; case 9 then // zero-crossing disp("Flag value: 9"); y1 = []; xnew = x; else // default disp("Unexpected flag value."); y1 = []; xnew = x; end endfunction ``` Then I have prepared very simple Xcos model ![Simulation_model](https://global.discourse-cdn.com/free1/uploads/utc/original/2X/2/23cc4efcfe3f4769fae0ced2d7b1fea3e37b2f53.png) The _scifunc\_block\_m_ parameters have been set according to the attachment. ![Block_parameters](https://global.discourse-cdn.com/free1/uploads/utc/original/2X/d/d2faadab61acfba8af15cc112863c367175cf92e.png) Unfortunately the simulation crashes. The diagnostic printing in the Console window occurs: “ARGS: 5“ and a warning occurs at the _scifunc\_block\_m_ right down corner. Namely there is “_Wrong number of input arguments.Expected 3._“ written. May I ask you for an advice how to implement this simple discrete system via the scifunc\_block\_m? ---
**Author:** ![mottelet](https://yyz2.discourse-cdn.com/free1/user_avatar/scilab.discourse.group/mottelet/32/13_2.png) [@mottelet](https://scilab.discourse.group/u/mottelet)\ **Post date:** [February 12, 2026, 9:33am UTC](https://scilab.discourse.group/t/how-to-implement-discrete-dynamic-system-in-xcos-v2024-1-0/1114/3 "2026-02-12T09:33:22Z")
Hello, I think you are mixing up scifunc\_block\_m and generic\_block3 (I recognize that generic\_block3 is not sufficiently documented and misses examples). Maybe @davidcl can confirm ? S. ---
**Author:** ![Student](https://avatars.discourse-cdn.com/v4/letter/s/ecc23a/32.png) [@Student](https://scilab.discourse.group/u/Student)\ **Post date:** [February 12, 2026, 1:25pm UTC](https://scilab.discourse.group/t/how-to-implement-discrete-dynamic-system-in-xcos-v2024-1-0/1114/4 "2026-02-12T13:25:41Z")
Hello, thank you for your reaction. I have placed the _scifunc\_block\_m_ block from the User defined functions category into my simulation. Why do you think I have used the _generic\_block3?_ ---
**Author:** ![mottelet](https://yyz2.discourse-cdn.com/free1/user_avatar/scilab.discourse.group/mottelet/32/13_2.png) [@mottelet](https://scilab.discourse.group/u/mottelet)\ **Post date:** [February 12, 2026, 2:08pm UTC](https://scilab.discourse.group/t/how-to-implement-discrete-dynamic-system-in-xcos-v2024-1-0/1114/5 "2026-02-12T14:08:36Z")
You have used a `scifunc_block_m` but your function prototype ```scilab [xnew,y1] = Digital_low_pass_filter(flag, x, u1, rpar, ipar) ``` has the argument list of functions used by the `generic_block3` block. Inside a `scifunc_block_m`, if you call your function, there is no mecanism that would give you values to `flag, x, u1, rpar, ipar` (unless you pass actual arguments by defining them explicitely). S. ---
**Author:** ![Student](https://avatars.discourse-cdn.com/v4/letter/s/ecc23a/32.png) [@Student](https://scilab.discourse.group/u/Student)\ **Post date:** [February 12, 2026, 3:58pm UTC](https://scilab.discourse.group/t/how-to-implement-discrete-dynamic-system-in-xcos-v2024-1-0/1114/6 "2026-02-12T15:58:26Z")
I see. May I ask you for an advice how to modify the function prototype to be in accordance with the _scifunc\_block\_m_? ---
**Author:** ![mottelet](https://yyz2.discourse-cdn.com/free1/user_avatar/scilab.discourse.group/mottelet/32/13_2.png) [@mottelet](https://scilab.discourse.group/u/mottelet)\ **Post date:** [February 12, 2026, 4:24pm UTC](https://scilab.discourse.group/t/how-to-implement-discrete-dynamic-system-in-xcos-v2024-1-0/1114/7 "2026-02-12T16:24:27Z")
can you attach your diagram to your post (the one with the screenshots) ? ---
**Author:** ![Student](https://avatars.discourse-cdn.com/v4/letter/s/ecc23a/32.png) [@Student](https://scilab.discourse.group/u/Student)\ **Post date:** [February 12, 2026, 8:50pm UTC](https://scilab.discourse.group/t/how-to-implement-discrete-dynamic-system-in-xcos-v2024-1-0/1114/8 "2026-02-12T20:50:58Z")
[Digital\_low\_pass\_filter.sci](https://scilab.discourse.group/uploads/short-url/eDRppvwgvsr7eYJfNKDxEn4zFXW.sci) (1.2 KB) [Digital\_low\_pass.zcos](https://scilab.discourse.group/uploads/short-url/nu1ZRGqUCUKPzt4fDP7sIGt8r2Z.zcos) (4.2 KB) I have just attached the Xcos model along with the function definition. ---
**Author:** ![Student](https://avatars.discourse-cdn.com/v4/letter/s/ecc23a/32.png) [@Student](https://scilab.discourse.group/u/Student)\ **Post date:** [February 17, 2026, 5:54am UTC](https://scilab.discourse.group/t/how-to-implement-discrete-dynamic-system-in-xcos-v2024-1-0/1114/9 "2026-02-17T05:54:49Z")
Do you see anything wrong in the Xcos model or the function definition? ---
**Author:** ![davidcl](https://yyz2.discourse-cdn.com/free1/user_avatar/scilab.discourse.group/davidcl/32/1029_2.png) [@davidcl](https://scilab.discourse.group/u/davidcl)\ **Post date:** [March 31, 2026, 9:22am UTC](https://scilab.discourse.group/t/how-to-implement-discrete-dynamic-system-in-xcos-v2024-1-0/1114/10 "2026-03-31T09:22:09Z")
Hello, After investiguation, it looks that `flag` is resolved to a Scilab macro. Calling `string(flag)` expect 3 output argument ; this is an issue. To have a better error report, I enable proper call stack when debug is enabled. You will get the expected error and stacktrace (with lines). See [Xcos: report Scilab intermediate errors in debug (!1680) · Merge requests · scilab / scilab · GitLab](https://gitlab.com/scilab/scilab/-/merge_requests/1680) ---
**Author:** ![davidcl](https://yyz2.discourse-cdn.com/free1/user_avatar/scilab.discourse.group/davidcl/32/1029_2.png) [@davidcl](https://scilab.discourse.group/u/davidcl)\ **Post date:** [March 31, 2026, 12:26pm UTC](https://scilab.discourse.group/t/how-to-implement-discrete-dynamic-system-in-xcos-v2024-1-0/1114/11 "2026-03-31T12:26:31Z")
In the dialog box of `scifunc_block_m`, the `flag` variable is not used by Xcos. ![image](https://global.discourse-cdn.com/free1/uploads/utc/original/2X/7/7e82520f7325b85155087127e3bc7f376cd74c0f.png) The `flag` variable you used here is the `flag()` function from colormaps. Thus the error. ---
**Author:** ![Student](https://avatars.discourse-cdn.com/v4/letter/s/ecc23a/32.png) [@Student](https://scilab.discourse.group/u/Student)\ **Post date:** [April 1, 2026, 8:25pm UTC](https://scilab.discourse.group/t/how-to-implement-discrete-dynamic-system-in-xcos-v2024-1-0/1114/12 "2026-04-01T20:25:20Z")
Thank you very much for your reaction. Does it mean that I have to remove the flag parameter from the function header? If so what is the correct approach for control of the _scifunc\_block\_m_ i.e. what is the mechanism how the solver interacts with the _scifunc\_block\_m_?