There are 4 - ZIPS slots. The unit comes with 4-256k Dynamic ZIP. This gives you 8 seconds on sample time. The unit can be upgraded to a maximum of 32 seconds, by swapping out all 4 slots with 1meg 20 pin ZIP 100ns or faster. These are the ONLY two Simm configurations this unit will run with. (All four simms must be swapped at once NOT in pairs. Either 4-256K's or 4-1m's, giving you the option of 8secs. or 32secs. of sample time.)