The ABS device (ABsolute non-Sense device) is an interface for Atari VCS system that allow to capture a defined game parameters, such as remaining lives, score and so on.
It's interfaced with cartridge, where the address-bus lines are routed to a PLD and data-bus lines are routed to external devices (i.e. microcontroller or latch chip...)
PLD is programmed in order to respond to a defined address value: when the address value programmed match the address value present on the bus of VCS, a PLD's output pin is set, so the external devices know that the preset address in on the way and they read the related data present on data-bus.
For example, in Pole Position game, at RAM address 88 (hex) is stored the car's speed; programming the PLD with preset address equal to 88 (hex), we can read the data associated to the car's speed (i.e. with a microcontroller interfaced with Atari data-bus and with the active output of PLD).
http://www.youtube.com/watch?v=kU65meFV_VY
In this case the car's speed is grabbed form Atari VCS memory ram , and it is displayed as led bar. The led bar value is equal to the speed-bar already present on the game.