![]() ![]() The emulator operates by converting a data block into the low-level signal required by Ethernet.Ī 0 bit is high-then-low and a 1 bit is low-then-high, with 170 nanosecond pulses. ![]() I'm using the BeagleBone's PRU microcontrollers to implement this code I wrote a blog post with more about the PRUs.Ī BeagleBone Black configured to emulate the 3Mb/s Ethernet on the Xerox Alto. These signals normally go to a transceiver, which converts these signals to signals over the network cable.) (The Ethernet signals to and from the Alto are 5V TTL. The photo below shows the BeagleBone, along with a chip (74AHCT125) to convert the BeagleBone's 3.3V signals to 5V TTL signals. I built an Ethernet emulator using a BeagleBone Black, allowing me to send Ethernet boot packets to the Alto. Unfortunately, it uses 3 Mb/second Ethernet over coaxial cable, which is incompatible with anything modern. The Xerox Alto was the first system with Ethernet, introducing a lot of networking innovations. The disk logic appears to work fine so hopefully next session we will be able to read and archive disks. (See his blog for details.) You'd think writing to RAM would be the easy part, but apparently not. Unfortunately, the read data was zeros, which was surprising since the Alto successfully read from the disk last week.Īfter some investigation, Carl found the problem was in the FPGA code that stored the data in RAM, not his code. It communicated with the drive just fine and could read from different tracks. We hooked up the FPGA board to the Diablo drive and tested it out. The FPGA board is connected to a prototyping board (right) with chips that shift the voltage levels to TTL as required by the Diablo drive.Ĭarl's FPGA code generates the numerous signals required by the Diablo drive in the photo below you can see the thick black cable going to the drive.Ī Digilent FPGA board configured to control a Diablo disk drive. This chip contains over ten thousand logic cells, allowing it to implement complex circuits. The FPGA board for the emulator (below) is a Digilent Nexys 2 with a Xilinx Spartan-3E FPGA chip in the center of the board. You describe the hardware configuration (gates, latches, and so forth) using a hardware description language such as Verilog and the chip is programmed to implement the desired circuitry. The FPGA chip contain numerous logic blocks along with a switch matrix that allows them to be interconnected as desired. ![]() If you're not familiar with a FPGA (field-programmable gate array), it is a chip thatĬan be programmed to generate custom hardware. In the photo above, the bottom oscilloscope trace shows several sectors as they are read from disk. It provides the raw bit stream as it is read off the disk, and the emulator needs to process this into bytes. ![]() Then we can connect the Emulator to the Alto and simulate multiple disk packs without physically handling disks.īuilding a disk emulator is complex because the drive itself implements very little functionality. The idea is we can hook this up to the Diablo drive to read and archive disks. On the right are the power supply and the laptop controlling the FPGA board.Ĭarl built a Diablo disk emulator / exerciser from a FPGA board. The Xerox Alto is behind the oscilloscope. Four sectors are visible on the bottom trace. The oscilloscope shows the sector pulses (top, blue), clock (middle, green), and data (bottom, yellow). Our test setup to exercise the Diablo disk drive (center) with the FPGA board (front). This session, we replaced the bad 7414 with a new one since we didn't want our hack to be permanent. Since we didn't have a 7414 lying around the house, we used a "dead bug" hack (below) to replace the bad inverter on the chip with an unused one, allowing us to access the disk. We discovered a faulty 7414 inverter chip on the disk interface card was preventing the disk from working: one of the six inverters on the chip had failed, preventing the disk sector task from running. Marc Verdiell, Luca Severini, Ron Crane, Carl Claunch and Ed Thelen.įor posts on previous restoration days see I'm helping restore the system, along with Y Combinator received an Alto from computer visionary Alan Kay and It introduced the GUI, Ethernet and laser printers to the world, among other things. The Alto was a revolutionary computer designed at Xerox PARC in 1973 The green dial on the right rotates to indicate the current track. Looking inside the Diablo disk drive, you can see the head moving over the disk's surface as disk seeks take place. ![]()
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