#mos6502

🎄💾🗓️ Day 14: Retrocomputing Advent Calendar - Atari 400/800🎄💾🗓️

The Atari 8-bit computer line was launched in 1979 with the Atari 400 and Atari 800. Both were advanced for home computers at the time. Both were driven by a 1.79 MHz MOS Technology 6502 CPU, with ANTIC and CTIA/GTIA custom chips for advanced graphics and the POKEY chip handling sound and input/output duties. The Atari 800 was the premium model, featuring a full mechanical keyboard, user-expandable to 48 KB of memory, more metal shielding, and more durability. The Atari 400 was a lower-cost alternative with a membrane keyboard, limited to 16 KB of RAM, and a simplified construction.

With hardware-accelerated scrolling, Player/Missile graphics, and rich sound, the Atari 8-bit systems were known for gaming and creative applications. They were a versatile platform with cartridge-based software, cassette, and floppy disk storage. Atari's proprietary SIO (Serial Input/Output) port allowed daisy-chaining peripherals such as printers, modems, and disk drives, making connecting them easier than with other systems.

Newer models were more compact, combined memory expansion to as much as 128KB, and compatibility with developing software and peripherals improved. Atari's 8-bit computers are remembered for having groundbreaking hardware and a very colorful game library, and they are still being used by the retrocomputing community.

While doing research for this, I saw the XE Game System, never saw it before, very 80s for sure!

The Atari XEGS (XE Game System) was launched in 1987. A repackaged 65XE with a removable keyboard, it boots to the 1981 port of Missile Command instead of BASIC if the keyboard is disconnected.

I’ve made great strides in fixing the issues with the front panel logic.

I figured out why the Memory Protect toggle and LED were malfunctioning.  Resistors can change alot.  So can checking your solder work for obviously disconnected bridge joints. I also fixed the Read/Write bus indicator, which was wonky as hell for awhile there. 

The step signal is now debounced correctly (I think), and the Run/Halt toggle seems to lock out certain front panel functions correctly, to prevent meddling with the bus while the CPU has been handed control.

Testing the complex sequenced features, like Examine, Examine Next, Deposit, and Deposit Next seems to be 70% working.  Deposit works fine, since it’s an un-sequenced event.

What’s not working is the “Load Data Bus” signal, which is the last action performed by all the three different sequence chains.  When the LDB input on the Data Control Card is pulled high with 5V, it works fine, and copies whatever byte is on the data bus into the front panel register for user manipulation.  However, when the Status Control Card is given control over that signal, the signal doesn’t approach 5V.... it goes from 0.25V up to 0.55V max, and then that’s it.

The MOnSter 6502