Depending on your application, Sealevel digital I/O products with either optically isolated inputs or TTL level signaling can be used.
Sealevel products with TTL inputs require tight coupling and will work for simple applications where there is a short distance between the input and output modules. TTL signals require voltages of 5 VDC.
Sealevel digital I/O devices with optically isolated inputs offer increased flexibly over TTL level inputs. The optically isolated inputs can interface over greater distances than possible with TTL signals. Optically isolated inputs are compatible with higher voltages (typically 5-30 VDC) and offer protection from ground loops. Each optically isolated input has an inline current limiting resistor and is non-polarized, so they can be wired without regard to the polarity.