4-Channel vs. High-density Digital SNAP I/O Modules

Most digital SNAP I/O modules are packaged with 4 channels in the module. High-density digital (HDD) SNAP I/O modules have either 16 channels or 32 channels. This table compares features for 4-channel modules and HDD modules. Download this table as a pdf.
Item SNAP High-density Digital Modules SNAP 4-Channel Digital Modules
Number of points on modules 16 or 32, depending on module 4
Isolation and fusing 16-point input modules: Each point is optically isolated from other points on the module.
32-point input and output modules: The module is divided into four groups of eight points. Groups are isolated from each other, but points within a group are not isolated from each other. Groups must be externally fused
Input modules: Each point is optically isolated from other points on the module.
Most output modules: Points are not isolated from each other. Points share a common fuse. For isolated modules, see the SNAP Digital Ouput Modules Data Sheet (form #1144).
Status LEDs None; use the handheld OptoTerminal-G20 for module diagnostics and commissioning, or for 32-point modules, connect to an optional breakout rack. One for each point, located on top of module
Polling time from I/O processor to module1  2–30 ms typical2 0.5–2 ms typical2
Module turn-on/off time1 16-point input modules: 15–20 ms
32-point input modules: 6 ms
Output modules: 100 microseconds
Varies by module. Examples:
- SNAP-IDC5-FAST: 25 microseconds
- SNAP-IDC5: 5 ms turn-on, 15 ms turn-off
Input latching Yes Yes
Counting on digital input modules Counting occurs on the module.3
Counting is available with SNAP-PAC-R14, SNAP-PAC-EB14, and SNAP-PAC-SB1 processors.
Counting speeds:
- On 32-point modules, 0–50 Hz @ 50% duty cycle
- On 16-point modules, 0–25 Hz @ 50% duty cycle
(Speed depends on module; see specifications for each part number.)
High-speed counting occurs on the I/O processor (brain or on-the-rack controller) and can be configured for any point. (High-speed counting is available on SNAP-PAC-R14, SNAP-PAC-EB14, and SNAP-PAC-SB1 processors.) Counting speed varies based on the processor and the speed of the module. Example: SNAP-PAC-EB1 brain with SNAP-IDC5-FAST: up to 20 KHz
Watchdog timer I/O processor firmware 8.1 and higher, yes.
Firmware 8.0 and lower, no.
Pulse generation Yes Yes
On-pulse, off-pulse, and period measurement No Yes
Frequency No Yes
Totalizer No Yes
Digital events I/O processor firmware 8.1 and higher, yes.
Firmware 8.0 and lower, no.
1 Actual turn-on and turn-off times equal the polling time plus the module time.
2 Polling time varies based on the SNAP I/O processor (brain or on-the-rack controller), processor configuration, and Ethernet host communication activity.
3 The high-density digital module uses a 16-bit counter, but the processor used with the module accumulates counts to 32 bits by periodically getting and clearing the module’s counts and adding to current values. Update time varies based on number of modules and Ethernet communication demands.
4 Including -FM (Factory Mutual approved) and -W (Wired+Wireless) models.