16-bit Digital Input/Output Signal Conditioning Plug-On. The VTI Instruments VT1533A 16-bit Digital I/O SCP provides 16 TTL compatible input/output lines. The 16 TTL bits are grouped as two, 8-bit "channels". Each channel can be configured as an 8-bit input port or an 8-bit output port. When configured for output, each channel can be either passive (resistor) pull-up or active (transistor) pull-up. Each output terminal is provided with over-voltage protection to prevent damage to the SCP or A/D module. Voltages greater than 6V on any terminal will generate an error.