Digital I/O board is a special type of data acquisition board, which contains digital inputs (I) and digital outputs (O). The digital I/O on a data acquisition board or card typically interfaces with standard logic-level digital signals. Digital I/O boards may have 4, 8, 16, or 32 lines or channels of digital I/O, and can be in most combinations of input and output channels. However, for more complex applications, standalone digital I/O boards are available with 96 channels of I/O. These special purpose boards might provide both logic-level and high current drive outputs, compatible with relay and SSR and relay racks. Digital I/O boards are available for most common bus configurations to meet most applications.