131 lines
5.7 KiB
Markdown
131 lines
5.7 KiB
Markdown
VAXstation 2000 SCSI Patched ROMs
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=================================
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The VAXstation 2000 is a fantastic little "shoe box" VAX computer. A little
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slow maybe but a great entry to the wider world of DEC's VAX computers.
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However it is somewhat lacking in one area: storage. It has one (optional)
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internal hard drive (mine came with a 150MB RD54) and one (optional)
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external hard drive - both of the MFM variety. That's not a big amount of
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storage, and old MFM drives are as you probably know prone to stiction and
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bad blocks, if they even work at all.
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The VAXstation 2000 does have a SCSI interface, however it was only intended
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to be used with the optional external TZK50 tape drive, and as such has never
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had any real support for adding hard drives, and it certainly cannot boot
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from a SCSI hard drive even if the operating system supported accessing one.
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But all is not lost. An experimental patch set for the VAXstation 2000 was
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created (but never officially released) to add full SCSI support to both the
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internal ROMs and to VAX/VMS, but only one specific version. Which is what
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we have here.
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Using the patches is not the easiest thing in the world, since you need
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an already working installation of VAX/VMS 5.5-2 (yes, that exact version)
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in order to run the patches. And if you don't have a working hard drive
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to operate the VAXstation on, how are you going to get a working installation?
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The answer is to net-boot it with a VAX cluster. But doing so makes it
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rather hard to then make a standalone image. But it can be done with a bit
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of shoehorning. Which thankfully we have done for you. In the releases
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section you will find a set of BlueSCSI hard drive images that should allow you
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to boot a VMS installation (dka200) that is ready to be completed with
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your own details, along with the installation source files (dka300) which
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the installation will ask you for.
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Using it
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--------
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In short:
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1. You need to upgrade your ROMs using the four ROM images (b1-b4) in the ROMs
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directory. You will need four new 64kB EPROMs (27512, 27C512, etc) to burn the images to then
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replace the ROMs on your VAXstation 2000's motherboard.
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2. Connect the BlueSCSI to the SCSI bus (important: see belo), making sure to enable termination if
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you don't have the TZK50 tape drive attached or an external terminator.
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3. Boot from the dka2 device:
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```
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>>> boot dka2
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```
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4. Complete the installation following the on-screen prompts. When it asks for
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the installation media tell it `dka300`.
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5. Install the patched binaries into the installed os:
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```
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$ mount/over=id dka300
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%MOUNT-I-MOUNTED, VMS552 mounted on _DKA300:
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$ set def dka300:[scsi.bin]
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$ @install
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```
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Now the long version:
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Installing the ROMs is pretty straight forward. There's 4 EPROM chips on the motherboard
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which need replacing.
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Note the order, as marked on these ROMs.
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The BlueSCSI (which should be a BlueSCSI II Desktop version so you get the proper 50 pin
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IDC connector) will not work with the VAXstation out of the box due to the VAXstation
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being from the dawn of SCSI and not working in quite the same way as other systems
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(the joys of old computers...) so the BlueSCSI will need to be modified in order to work.
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The problem here is that the SEL pin and RST pin of the SCSI are weakly tied together inside the
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BlueSCSI, so when the RST pin is asserted by the SCSI initiator the SEL pin gets pulled
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low, which shouldn't happen. Most things don't care - but the VAXstation does, and when that
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happens it just gives up trying to do anything on the SCSI bus ever again. So we need
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to separate those two signals. This needs some careful SMD rework.
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1. Remove the resistor R66. This is the resistor that ties the two signals together.
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2. Connect the left hand pad (while the text of R66 is in the normal orientation)
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of R66 to the pin GPIO17 of the Raspberry Pi Pico. This
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gives the MCU direct control over the SEL pin.
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3. Install modified firmware into the Pi Pico which moves the SEL functionality to
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GPIO 17.
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<img width="466" height="211" alt="BlueSCSIPatch" src="https://github.com/user-attachments/assets/802cbc62-8243-49b3-af2c-86bdc8daa508" />
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This modification uses one of the pins that was used for I2C for the SEL pin, so you will
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no longer be able to use the I2C connector on the BlueSCSI.
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For the curious
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---------------
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If you're curious about the steps needed to create the patched VMS images, here's what
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we had to do:
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1. Create a VAX/VMS 5.5-2 cluster using a SIMH emulated server.
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2. Enrol the target VAXstation 2000 into the cluster as a member.
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3. Patch the system using the original patch set
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4. Verify that SCSI is working and create two blank images on the BlueSCSI (id 2 and 3).
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5. Create a standalone backup kit on DKA300
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6. Copy the VAX/VMS-5.5-2 installation backup files onto DKA300
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7. Copy the SCSI patches to DKA300
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8. Boot from DKA300 and install the VAX/VMS base system to DKA200
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9. Reboot from the network and manually copy the patched files to the
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correct places on DKA200
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As you can see it's not a simple operation.
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One last word
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-------------
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Good luck!
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Postscript
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==========
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The patches do not calculate the checksum for the ROMs. This means you will get the VAXstation
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failiing at bootup with an error 5?? with `SYS 0000.0002`. Work is going on to rectify this, including
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reverse engineering of the checksum algorithm and the system test 5 to work out what is needed. Watch this space.
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## Update:
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The ROMs have now been updated with the correct checksums. It turns out the checksum is calculated per physical ROM,
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and is a simple rotate-left-and-add method. I wrote a [little program](https://github.com/MajenkoProjects/vaxcheck) to calculate the checksums for me.
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