IMSAI - RAM 4A Card
This was an update of IMSAI's first 4K RAM board. It was a well designed board and had nice features like write protect and memory access LED's

The IMSAI RAM 4A board provides up to 4K bytes or static random access memory.  The board was implemented with 2102-style memory chips that each have the capacity to store 1024 words of one bit for each word.  Thus eight chips are used to store one block of 1024 eight-bit words. Up to four sets of eight-chip units were used on the board, giving a maximum capacity of 4096 eight-bit words.
 
Each eight-chip unit had the circuitry to allow or prevent the ability to write information into their memory storage space.  This "write-protect" feature could be controlled either by software commands or from the computer front panel.  Software commands could both affect the write protect and test the status of the write protect.  If a program attempted to write into a write-protect block of memory, an interrupt was generated.  (This feature may be disabled if desired.)  Four red LED's are provided to indicate the protect status of each of the 4K blocks of memory.  Four green LED's are also provided which illuminate when their respective block of memory is addressed. The RAM 4A board supported a front panel write protect switch.  If the machine was stopped, the 1024 word block at which the machine address is pointing will have its memory write protect status affected through the use of a PROTECT/UNPROTECT switch on the front panel.  Attempts to write into this section of the memory will, of course, not succeed. The RAM 4A board was designed to allow the user to provide battery backup power.  Trickle-charging facilities to allow the battery to be charged while the computer is running may also be installed on the board by the user.   
 
The manual (updated in 2002) for this board can be obtained here.