The 6ES5998-5SD21 is a plug-in I/O module from Siemens, part of the Simatic S5-100U PLC system. It has 8 universal digital inputs, 8 universal digital outputs and 2 relay outputs. The inputs are configurable as either digital or analogue, and the outputs can be configured to supply voltage and current in a variety of formats. The I/O module connects to the S5-100U motherboard via a 24 pin ribbon cable. All signals are optically isolated, allowing for debugging, access to diagnostics and configuration data.