I am building a simulation of an old 70s design based on 74161s and I have had a lot of trouble getting a set of counters to operate according to my expectations.
In my search for understanding more I discovered a recent document that had identified an error in the 74161 multisim model related to the counter executing on falling edge of clock, should be rising edge . Unfortunately it did not solve my headache.
Essentially I have three 74161s in cascade - the RCO for the first counter nicely carries over via ENT to trigger the second counter (ENP is high). But, when the second counter's RCO carries over to the third counters ENT it stays high for a long period of time (the duration of the QA) and sets off the third counter to "race at clock speed" similar to the first counter. When I do the same thing but instead carry over to ENP (having ENT high all the time) the counters work as expected.
The old schematics I use as a base, as well as some cascade counter reading I have come across indicates RCO should connect with ENT and not ENP – then again reading the 74161 datasheet it states that RCO stays high during the duration of QA.
Question to the forum: is the model for 74161 in multsim even more incorrect than the clock edge issue or is my “three cascaded counter” design incorrect? Please find attached the multisim model.
I appreciate any insights.
Is this issue still active/unresolved?
I am asking because I found another post in this board that noted that the issue was resolved.
Thanks in advance for your feedback!
thanks for engaging.
I am new to NI tools and new to the forums.
I orginally posted in the wrong forum and tried to move the entry, or at least not cause the entry to engage more people in the wrong forum by setting it as resolved in the mean time.
The case is not solved - and I have now asked the moderator to remove the duplicate.
Thank you for your feedback!
Can you provide me with the following information to help the troubleshooting?
- Is there any difference between the two files you posted?
- Can you provide me with the original reference document on which you based your design?
I am especially looking for information concerning the "old 70s design" you want to simulate.
The more (clear) information I have concerning this, the better I can understand what might be going wrong in/with the simulation.
- Which version of Multisim are you using?
Please provide me with a screenshot of the "Help>About Multisim" Windows and do blur out your serial number (if it is mentioned over there)
- Which simulation settings are you using?
Thanks for your reply and apologies for my late return on this.
Please find attached a zip file with the requested information and with updated examples files
* One case where RCO from counter is connected to ENP and one case where RCO from counter is connected to ENT. When connected to ENT (as it appears to be done in the orignal schematics p21, B4 to E4) the third counter starts to race at clock speed when RCO set ENT high (as can be seen in the attached simulation screen shot). This is not the case when connecting to ENP. Something wrong with the 74161 model?
* https://archive.org/download/ArcadeGameManualComputerspace/computerspace.pdf The pdf contains the schematics - see page 21, circuits B4 to E4. They should form a simple 16 bit binary counter that keeps incrementing until all of the output bits for E4 are all zeros and then it loads all the counters with preset value. In the schematics the rco is connected to ENT.
* My test cases (rco to enp and rco to ent) just simulates the circuits B4 to D4 - because it is at D4 where the issue occurs first in the chain (it also occurs at E4 - but not need to simulate; it is the same isse)
Any insight is highly appreciated.
For some reason i couldnt attach files with my message above, and i cannot post them separately either it appears...
I will try tomorrow again
I tried to simulate the similar 74161 cascade in another simulation tool - and i get the reverse (and expeced) results; that when RCO is connect to ENT, the carry is forwarded and no "racing" occurs.
So this appears to indicate that the 74161 model in multisim appears to have shifted the ENP and ENT logic,
My apologies for not getting back to you earlier.
I didn't seem to receive the appropriate updates/notifications.
Thanks for sharing these files over here!
Some remarks/feedback from my side:
- I am using a slightly different version of the software on my pc than on your pc.
With this set-up it does execute on the rising edges.
- Do note that your simulation is using a 74161N and not a 74161.
- Aside from this, I have to confirm that I can reproduce the behavior that you are seeing.
Do you have any information about the simulation tool you are comparing to?
Are they using models of a 74161 or a 74161N in there?
Is the model that is being used in there shareable?