Supported System Software: 7.1, 7.1.1(Pro), 7.5, 7.5.1, 7.5.3
SCSI: DB-25
Serial Ports: 2
Sound Output: stereo 16 bit.
ADB: 1
Ethernet: AAUI-15 built-in
Machine Gestalt ID: 78

Minutiae:

Apple sometimes advertises this as an 80 MHz machine, since the clock speed is doubled inside the processor chip.  However, all I/O to the processor is done at 40 MHz. 

Upgraded with one SIMM at a time.

It does not support 1MB, 2MB SIMMs.

The Quadra 840AV contains the 66MHz AT&T 3210 DSP chip. 

Serial Port with the phone icon supports synchronous modems, with handshaking on both both output and input supported.

Has 2MB ROM and a 1.44MB SuperDrive.