Incorrect Simulation of TTL Level Switching Logic, TTL Behaves like 5V CMOS

Updated Apr 1, 2018

Reported In

Software

  • Multisim

Issue Details

I added a Tiny Logic NC7ST04_5V to my Multisim schematic. This is a CMOS device with TTL-compatible inputs, specified to interpret an input voltage <0.8 V as low and >2.0 V as high. In reality, however, these devices switch at about 1.5 V.

When I place a supposedly TTL-compatible chip in Multisim I expect it to switch at the "correct" voltage of 1.5 V. However, the device switches at Vcc/2 which is 2.5 V. It behaves just like a regular 5 V CMOS part which gives wrong results for input signals between 2.0 V and 2.5 V!

Solution

Multisim features two different simulation models for device pins: an Ideal Pin model for faster simulation results and a Real pin model for more accurate simulation results. With the former option Multisim uses Vcc/2 as switching level for all inputs. Set to the latter it uses the realistic switching levels.

You can find this setting in the menu Simulate » Mixed-mode simulation settings:

Additional Information

When using Real pin mode make sure to have the proper power rails on the circuit. It's not always the same for each component. One can see which one(s) a component needs in the Pins tab of the component properties:

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