IMSAI - PIO Board
This board is found in almost all the early IMSAI systems.   It is a 4 parallel port I/O board

The PIO 4 board provides for up to four input and four output ports of eight bits each parallel input and parallel output.  Each input and each output port had its own latch and both input and output latches were provided with hand­shaking logic for conventional eight bit parallel transfers. Connection to the input or output ports is made through board edge connectors at the top of the board on -10 inch centers and the fingers will accept the 3M flat cable edge connectors as well as most other .1  inch center-to-center board edge connectors. The handshake logic on any input or output port will generate an interrupt. The priority level of the interrupt is selectable.  The address of the four ports is four sequential addresses, and this block of four addresses may be jumper-selected to be any block of four sequential addresses in the 256 I/O address space.  The board may also be addressed with memory-mapped I/O, in which case normal memory read or write instructions are used to read or write data to the Input/Output ports.  When using memory-mapped I/O, board addressing is done by selectable jumpers for the lower byte of address and the upper byte of address is hex FFH. Provision was made for each of the four output ports to drive eight LED's for a total of 32 on-board LSD's. This feature can be used to provide program-controlled out­put for dedicated processor applications of the IMSAI 8080 in which case this PIO board would be plugged in where the front panel would normally be mounted and a special photo­graphic mask made to put in front of it with the appropriate labels for the specific purpose the controller is to be used-  The front panel can still be used during development by plugging it into an extender card in another slot.   The board utilized 8 Intel 8212's. 
 
The LED's are a nice feature on this board. I use one port to sequentially light up LED's in a bit pattern as my monitor initializes different components of my S-100 system. Great for debugging if your system hangs without a working monitor.
 
The manual for this board can be obtained here, the schematic can be obtained here.