The SNAP-D4M I/O mounting rack is designed for discrete control applications and can accommodate four SNAP 4-channel digital modules. The rack has an industry-standard 50-pin header connector, which allows it to be used in a variety of applications.

The logic side of the I/O circuitry can be controlled in one of several ways:
- By a Raspberry Pi®, using Opto 22's Digital I/O Carrier Board for Raspberry Pi (part number OPTO-P1-40P)
- By a PC, using an Opto 22 PCIe-AC5 (PCI Express bus), PCI-AC5 (PCI bus) or G4AC5 or AC5 (ISA bus) PC adapter card.
- Through an Opto 22 Classic brain board, which uses one of Opto 22's industry-standard protocols (mistic®, Optomux®, or Pamux®) to control the I/O and communicate either serially or in parallel

Field devices are wired directly to the top-mounted removable connectors on the SNAP I/O modules. The module and rack design allows modules to simply €œsnap€ on and off the mounting rack. Use two 4-40 by ½-inch standard machine screws to secure each module in position.

All SNAP racks offer panel mounting and the option of DIN-rail mounting. SNAP racks use a single 5 VDC power source.