Apple Mac Performa 640 series - Manual (Page 149)

4 Align the card over the communication slot.
5 Press down on the card until the connector is solidly in place.
Don’t force the card. If you meet resistance, pull the card out and try again.
To see if the card is properly connected, lift it gently. If it resists and stays in
place, it’s connected.
Installing or replacing a SIMM for the PC environment
Your computer may have come with a SIMM (memory module) already
installed on the DOS-compatibility card inside your computer. The SIMM
provides memory for the exclusive use of the PC environment. (See the
Technical Information booklet to find out whether your computer has such a
SIMM installed.) To replace the SIMM with a SIMM of greater capacity, or to
install a SIMM if you don’t already have one installed, follow the instructions
in this section.
Note: The SIMMs used by the DOS-compatibility card are the same
72-pin SIMMs used to expand memory in your Macintosh. They are
interchangeable.
Communication slot
Vertical plate
Communication
port access hole
141 Installing an Expansion Card or Memory for the PC Environment