The RTC-BUF50 is a TTL-level digital input/output expansion board with 48 TTL-level I/O lines that are selected as six 8-bit I/O ports. These six ports may be configured as any combination of inputs and outputs. Two industry-standard 50-pin ribbon cable headers provide external connections to typical Opto-22-type expansion interface boards such as the IDI24 (optoisolated DC input) and IDO24 (contact-closure relay output).