Apple Performa 6400/180 - Manual (Page 40)

Basics Dual In-Line Memory Modules (DIMMs) - 38
Important:
The SIMMs used in previous Macintosh models
are
not
compatible with the 6400 or 6500. For the
Performa 6400, use 168-pin, fast-paged, 70 ns or faster,
2K refresh rate DIMMs. For the Power Macintosh 6400, use
168-pin extended data out (EDO) mode, 60 ns or faster, and
1K or 2K refresh count. For the Power Macintosh 6500, use
168-pin, 5-volt buffered extended data out (EDO) mode, 60
ns or faster, 2K refresh rate DIMMs. Do
not
use DIMMs with
a 4K refresh count.
Also, the JEDEC MO-161 specification shows three possible
heights for the 8-byte DIMM. For these computers, use only
the shortest of the three (1.100 inches). Taller DIMMs put
excessive pressure on the DIMM sockets due to mechanical
pressure inside the case.