Lighthouse PLCs, Inc. : Automation Forum
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Dear Tony Nardella,
Thank you for your interest in Hitachi PLCs.
Let’s try to answer your question by looking at the HSC table in the Micro-EH Users
Manual.
Next, let’s look at the math:
Since the symbol f
is used for frequency and the symbol T is used for period, these equations are also expressed as:
Therefore, the HSC (minimum) size Count Pulse Width is 100µS (T).
This would result in the maximum frequency of 10kHz(f).
Although this is a “built-in” counter, it is designed to react much like a "hardware"
HSC. Normal counters are typically “software” counters. In other words, they don’t physically exist in the PLC but rather they are simulated in software. As hardware counters do exist in the PLC, they are not dependent on scan time. A good rule of thumb is simply to always use the normal (software) counters unless the pulses you are counting will
arrive faster than 2X the scan time. (i.e. if the scan time is 2mS and pulses will be arriving for counting every 4mS or longer then use a software counter. If they arriver faster than every 4mS (3mS for example) then use the hardware (high-speed) counter (2 X scan time
= 2 X 2mS = 4mS).
Conclusion: Since the Micro-EH processor runs at 0.9µS/command, and you have 3K step maximum FLASH memory (assume completely full - which is highly unlikely), your scan time would be approximately 2.7mS (0.0000009 X 3,000 = I hope this is what you were looking for, and that I have answered your question. --- --- --- --- --- --- --- --- --- Replying to: We are looking at the EH Micro PLC .Can you tell us on the high speed counter input of 10Khz what is the shortest pulse we need to use to make it count.
0.0027 Seconds (T)). Two times that (rule of thumb above) would be 5.4mS. So if the period (T)
of your pulse is less than 5.4mS (f >185Hz), you would be well advised to take advantage of the HSC function provided for you in the Micro-EH.
Timothy Davis 
Regards
Tony
