Question about 555 timer chip in few 2600 consoles

[7800fan]

Having taken apart many 2600 consoles for repair (all NTSC), I have seen only one from a Darth Vader console that had extra chip not in schematic, a MC1455 which is Motorola's 555 timer chip. A quick examination shows the output goes to RIOT chip, and there's no external trigger or anything so it is self-running chip. I'd have to trace it to figure out exactly which pin.

 

As I haven't seen it in any schematic I've looked, I have no idea why there's one or why it only appeared in a later 4 switch model and not in Jr. Any of you know anything about 555 timer in 2600 console?

[Osgeld]

the vader I have for sale has one in it, thought hm interesting but I havent bothered to see what its actually doing, I would guess some form of debounce circuit

[7800fan]

Traced it, identified 3 parts and the chip it's connected to. It seems like it triggers only once on power on to reset the CPU?

 

I wonder if there was a design variation that broke CPU when Atari redesigned the motherboard for 4 switch system that was fixed in Jr model? Perhaps a quirk of CPU variation?

[Osgeld]

I dont think I have seen it in a 4 switch woody, and I have had a sunnyvale 4 switcher

[zylon]

In the field manual, it says that timer IC was put in to reduce "power on, reset failures". Rev 17 and 18 have it, maybe 16 as well.

[+DrVenkman]

In the field manual, it says that timer IC was put in to reduce "power on, reset failures". Rev 17 and 18 have it, maybe 16 as well.

 

I just checked disassembly photos of my Rev 16 Vader - it has the 555 chip as well.

 

[ulay@hotmail.com]

Mi Atari is a 2600 SECAM (France) model. It has the same IC. It is common in computer systems to provide for a safe boot up mechanism for the processor. If the processor is fast and tries to boot immediately it is turned on, it could be that power rails, ic statuses, etc, have not stabilized, the processor can get confused and crash. So a delay between when initial power is applied and when the CPU is allowed to boot is put in place to avoid this.