Description:
If 1769-IF4 is used to gether very fast analog input signal, should check the module update time to see if your configuration matches your requirement.
Calculations:
According to the user manual:
“Module update time can be calculated by adding the sum of all
enabled channel times. Channel times include channel scan time,
channel switching time, and reconfiguration time. The module
sequentially samples the channels in a continuous loop.”
Channel Update Time
| Filter Frequency | Channel Update Time |
| 50 Hz | 22 ms |
| 60 Hz | 19 ms |
| 250 Hz | 6 ms |
| 500 Hz | 4 ms |
Switching and Reconfiguration Times
| Description | 50 Hz | 60 Hz | 250 Hz | 500 Hz |
| Channel Switching Time | 46 ms | 39 ms | 14 ms | 10 ms |
| Channel Configuration Time | 116 ms | 96 ms | 20 ms | 8 ms |
Sample Calculation:
If all 4 channels are setup at 500Hz, 0-10VDC, there is not channel reconfiguration time.
Each channel update time = channel scan time (4 ms) + channel switching time (10 ms) = 14 ms
Total Module Update time = 4 x channel update time = 64 ms.
if one of the channel’s configuration is different, you need to add the channel configuration time twice, one for entering the channel and one for leaving it to enter the next channel. for 500 Hz, the penalty is 16 ms (8 ms x 2), which isn’t very bad.
Recommendations:
- Disable unused channels.
- Group same configuration Inputs to the same card to avoid reconfiguration time. Or at least group same configuration channels to next to each other to minimize reconfiguration time.
Reference: Allen-Bradley 1769-IF4 User Manual