Monday, May 30, 2016

Working with ST-506 Interface MFM Hard Drives

If you have an IBM PC, XT or compatible of similar vintage, historically accurate options for mass storage can be a bit tricky to work with.  No XT-IDE, compact flash or Disk on Modules were available during the first decade after the release of the IBM PC.  Drives were huge but storage capacity was small by today's standards.  Still, if you want to go 100% Oldskool PC, you should use a vintage drive.  Even a vintage hard drive is far superior to being relegated to floppies.

The first hard drive interface in the PC compatible world came with the IBM PC/XT.  The XT included a "fixed disk drive" controller designed by Xebec.  The controller used an interface called the ST-506 after Seagate ST-506 hard drive.  The ST-506 was a 5MB drive and used a two-cable interface.  The IBM original controller only officially supported one type of hard drive in its first two iterations, the ST-412.  The ST-412 functioned like the ST-506 but had a 10MB capacity.

Later, Seagate released the ST-225, a 20MB hard drive that could be found in late model XTs and perhaps the IBM PC XT/286.  IBM released a final revision of its fixed disk controller to support this drive.

MFM drives can take up a full-height 5.25" drive bay.  These bays are seldom found outside the original IBM PC, XT and XT/286.  The ST-412 is a full-height drive, the ST-225 is a half-height drive.

Thursday, May 12, 2016

The RetroUSB AVS - A Potentially Worthy FPGA NES HDMI-Output Clone

This year, bunnyboy (Brian Parker) of is going to release his long-awaited (if you are a NintendoAge forum member) AVS.  The AVS is a clone of the NES done within the programmable logic of an FPGA.  It comes in a NES-front loader influenced case, has a front loading 72-pin connector (no push down tray) and a top loading 60-pin connector for NES and Famicom games, respectively.  It only outputs HDMI at a 720p resolution.

The FPGA is a hardware recreation of the internals of the NES, namely the 2A03 CPU and the 2C02 PPU, the RAM and the glue logic required for a functioning NES.  An FPGA is a large, programmable surface mounted chip which allows the programmer to define the logic elements on the chip.  In this case, the programmer is attempting to model the CPU and PPU chips to perform an identical function to the logic contained in the discrete, through-hole chips Nintendo used.  Fortunately, these chips have been decapsulated and their dies have been imaged at very high resolution.  How they work on the hardware level is reasonably well-known, although there are some minor variations between the various revisions of each chip.

Sunday, May 8, 2016

IBM's CGA Hardware Explained

The IBM Color/Graphics Card has been widely seen as a poor attempt at a video adapter.  Released with the IBM PC back in 1981, it was not particularly impressive by the standards of its day. Limited colors and no sprites did not make it very attractive for games.  However, when you look at the hardware and what it could do, it becomes more impressive.  Even though the card is a full length card, it was built from off the shelf logic chips, memory and video controller.  Looking at the hardware also helps one understand the limitations of the device.

The BIOS Modes

Mode 00h - 40x25 B&W
Mode 01h - 40x25 Color
Mode 02h - 80x25 B&W
Mode 03h - 80x25 Color
Mode 04h - 320x200 Color
Mode 05h - 320x200 B&W
Mode 06h - 640x200 B&W

On an RGBI monitor, the identical Color/B&W modes have no distinction except in Modes 04 and 05. On a color composite monitor or TV set, color modes enable the color burst and b&w modes disable the color burst.  The IBM PC defaults to the 40x25 or 80x25 B&W modes depending on how you set a dipswitch.  Text, especially 80-column text, is much more difficult to read on a composite color display.

The CGA card has 16KB of RAM.  A full screen of 40-column text required 2KB of memory, allowing for 8 separate pages.  A full screen of 80 column memory required 4KB of memory, allowing for 4 separate pages.  Graphics modes took up all the 16KB of memory.  In order to really put the CGA card to work, one has to go deeper and look beyond the BIOS and what could be done by accessing its registers directly.

Tuesday, May 3, 2016

Metro ED500 DataVac - A Verse Review

The Metro ED500 DataVac 500-Watt 120 volt 0.75-HP Electric Blower Duster :

It will blow quite strong,
It will blow rather long,
Enjoy someday the money you will save,
No cans burying you into an early grave,
With nifty attachments to spare,
Away will fly the dust and hair,
Computers and gadgets clean up swell,
But it heats up like hell, and "burnt rubber" fairly conveys its smell.