The IMSAI RAM 4A board provides up to 4K bytes of static random access memory.
The board is implemented with 2102-style memory chips that each have the capability 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 can be used on the board giving a maximum capacity of 4096 eight-bit words.
Each eight-chip unit has the circuitry to allow or prevent the ability to write information into their memory storage space.
This “write protect” feature can be controlled wither by software commands or from the computer front panel.
Software commands can both affect the affect the write protect and test the status of the write protect.
If the program attempts to write into a write-protected block of memory, an interrupt will be generated.
(This feature may be disabled if desired.)
Four red LEDs are provided to indicate the protect status of each of the 1K blocks of memory.
Four green LEDs are also provided which illuminate when their respective block of memory is addressed.
The RAM 4A board is no longer in production.
The primary reason for the retirement is that the 2102 RAMs are very scarce and expensive.
ATTENTION: The green LEDs on the RAM 4A are more than just indicators.
They are part of the pull-up circuitry on the open-collector RAM bank select lines.
If they are completely out or even dim, you should replace them.
IMSAI has new green LEDs in a set in the table below meant to replace all of the LEDs.
The kit also includes spacers for easy assembly and a neat appearance.