I understand that you have a discrete multirate model (two signals: one at 2 Hz, one at 0.5 Hz) that then both go through ZOH blocks to a 1 Hz signal. When the configuration parameter “Treat each discrete rate as a separate task” is not enabled, the model runs fine. When you do enable that parameter (to support code-generation/autocoding workflow) the build fails with an error about a “sample time 1 of … at input port 1 is different from sample time 2 … at output port 1” for the ZOH block.Moreover, a Rate Transition block is automatically inserted for the 0.5 Hz→1 Hz branch, and the signal gets delayed (by 2 s) via that Rate Transition.
The main concern is: Why is that automatic Rate Transition/delay being introduced when enabling the “separate task” option? And is there any way to prevent the signal delay while still enabling “Treat each discrete rate as a separate task”?
When you enable “Treat each discrete rate as a separate task”, Simulink switches from single-tasking to multitasking execution.
Each unique discrete sample time (0.5 s, 1 s, 2 s, etc.) is now run in its own task with its own scheduling and priority.
In multitasking mode, two blocks running in different tasks cannot exchange data directly — doing so risks race conditions and nondeterministic execution.
To maintain data integrity and deterministic timing, Simulink enforces that data flowing across task boundaries go through a Rate Transition block (explicit or automatically inserted).
In your model:
- The 0.5 Hz → 1 Hz path is a slow → fast rate transition.
- The slower task updates its output only every 2 s, while the faster task (1 Hz) executes every 1 s.
- Therefore, the faster task reads the last available value from the slower task until a new value arrives — effectively introducing a one-slow-sample (2 s) latency.This delay is not a bug but a direct consequence of deterministic data transfer in multitasking systems.
How to fix or control the behavior
You have a few options depending on your goal:
- Keep multitasking and fix properly
- Go to Configuration Parameters → Solver
- Enable Automatically handle rate transition for data transfer
- Set Deterministic data transfer to one of the following:
- Always or Whenever possible → fully deterministic (safe but with delay)
- Never (minimum delay) → minimizes latency but may be nondeterministic
- This allows Simulink to automatically insert and configure the necessary Rate Transition logic.
2. Manual control
- Insert an explicit Rate Transition block between the 0.5 Hz source and the 1 Hz ZOH input.
- Check Ensure data integrity during data transfer.
- If you want to reduce the perceived delay, uncheck Ensure deterministic data transfer.
- (Optional) Set the block’s Initial condition to your desired starting value.
You can refer to the following MATLAB documentation for more details:
