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Thread: What did I do to my PDP-8 today.

  1. #111
    Join Date
    Sep 2020
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    Perth in Western Australia
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    I have succcessfully booted OS/8 via Kyle Owen's SerialDisk via a M8655 running at 19200 bps.

    I also found that my terminal connection using the M8650 running at 110 bps is stretching my patience too far.
    The crystal on the M8650 is 14.418 MHz so limits me to 110 bps. If I understand correctly by upgrading the crystal to 19.661 MHz I could run at speeds up to 2400 bps which is more tolerable.
    Has anybody tried this crystal upgrade?

    Thanks and best regards
    Tom Hunter

  2. #112

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    I have the 19.661 MHz crystal in mine. Faster baud rates work fine you just have to solder wire to IC pin. The M8650 can go faster than the M8655.
    https://homepage.divms.uiowa.edu/~jo...rd8e/kl8e.html

  3. #113
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    Jun 2012
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    Excellent work Tom.

    Dave

  4. #114
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    Sep 2020
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    Quote Originally Posted by djg View Post
    I have the 19.661 MHz crystal in mine. Faster baud rates work fine you just have to solder wire to IC pin. The M8650 can go faster than the M8655.
    https://homepage.divms.uiowa.edu/~jo...rd8e/kl8e.html
    Thanks Doug,

    I found a possible local supplier of the crystal and I hope I will get it quicker from Melbourne/Victoria than via Digikey or Mouser in the US.
    Your link shows that the M8650 can go much faster than the M8655 but common USB to serial converters won't be able to use the speeds over 38400 bps.
    Nevertheless this is twice as fast as the 19200 bps I am using with SerialDisk now.
    I will use the M8655 for the terminal connection and the now higher speed M8650 for SerialDisk.

    Best regards
    Tom Hunter

  5. #115

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    djg = David Gesswein. Added my name to profile.

    If you want 57600 or above I think that works with 14.7456 MHz crystal. Won't be able to do the common lower frequencies but if dedicated for SerialDisk won't mater.

  6. #116
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    Quote Originally Posted by djg View Post
    djg = David Gesswein. Added my name to profile.

    If you want 57600 or above I think that works with 14.7456 MHz crystal. Won't be able to do the common lower frequencies but if dedicated for SerialDisk won't mater.
    Sorry David,

    You linked to Doug's article so I wrongly guessed "djg" is Doug.

    For the higher speeds to work I would need power of 2 multiples of 110:

    57600 / 110 = 523.64

    523.64 / 512 = 1.023

    This means there is a 2.3% error but that is fine for async comms.

    57600 bps is a substantial improvement on the 38400 possible wiith the 19.661 MHz crystal upgrade.

    Actually 115200 bps should be equally possible with the same bitrate error.

    Thanks for pointing out this possibility.

    Best regards
    Tom Hunter

  7. #117
    Join Date
    Oct 2019
    Location
    Rapid City, SD USA
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    Grats on getting serial disk running!
    Quote Originally Posted by thunter0512 View Post
    Thanks Doug,
    It wasn't me.
    Quote Originally Posted by thunter0512 View Post
    Your link shows that the M8650 can go much faster than the M8655 but common USB to serial converters won't be able to use the speeds over 38400 bps.
    Nevertheless this is twice as fast as the 19200 bps I am using with SerialDisk now.
    I will use the M8655 for the terminal connection and the now higher speed M8650 for SerialDisk.
    The limiting factor on the M8655 is the UART. Some of them will work at 19200. The M8650 with a different crystal could go a lot faster.

    I have been working on making a parallel port disk. I have the OS/8 handler written. I was going to just modify Kyle's Serial Disk server but there were all kinds of changes to it I wanted to make. Most of those changes have to do with support for 12 devices in the SYS handler and performance changes. Performance is irrelevant when you use a low baud rate serial port but it is possible to do a lot better than that when using a parallel port. It looks like about the worst that could be expected would be around 55k words per second (82.5k bytes per second or 660k bps). I am hoping to see speeds of over 100k words per second. For comparison I believe there was a drum device that would transfer a word every 16us which would be a rate of 62500 words per second. The current version of serial disk if running at 19200 baud would transfer 960 words per second.

    Anyway, working on parallel disk is several projects down the list.
    Doug Ingraham
    2nd owner of Straight 8 SN1173
    5 other PDP-8's including an 8/i and a DECSet 8000
    SOL-20

  8. #118

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    I finally fixed the operation of
    *.-1 177+1
    in my cross assembler. I had to completely redo expression handling so that it processes left to right, etc.

    Also ran into a curious bit assembling the source for 8BAL (decus-8-497). The correct answer for this:
    SZA CLA^2
    is "SZA CLA" times 2, or 7500. But this:
    JMP FOO+1
    in the case where FOO is offpage, is very definitely *not* the same as "JMP FOO" + 1!
    Fun times.

    I've been working to close about a hundred tabs that I opened when upgrading the "DEC" and "DECUS" software collections on my website. (One of them was the links to the source for 8BAL and 8BALIB.) 8BAL uses quite a few exotic constructs that the cross assembler wasn't dealing with correctly.

    I also fixed a bug where "(" and "[" literals went to separate pools that didn't know about each other, in the case where "(" was used while on page zero (which generated a "page zero" message, but also gave wrong code).

    I haven't committed the new assembler yet, through, as I want to see it work correctly for more sources, first.

    Vince

  9. #119
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    Rapid City, SD USA
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    Quote Originally Posted by vrs42 View Post
    Also ran into a curious bit assembling the source for 8BAL (decus-8-497). The correct answer for this:
    SZA CLA^2
    is "SZA CLA" times 2, or 7500.
    I can't think of why you would ever want to do that. In what context does that make sense? A 7500 is an SMA. Is this some kind of code obfuscation?

    Ok, I looked at the source and I still don't understand it. There is a table of these with magic constants. 8BAL is a macro processor. The comment has to do with making an assumption that if the code is known then it is probably not an 8BAL IF construct. Somehow they match the SZA CLA with the 7500.

    Vince, is there a document describing 8BAL? I didn't see more than just the broad DECUS overview.

    (See how easily I am distracted?)
    Last edited by DougIngraham; January 17th, 2021 at 07:44 PM.
    Doug Ingraham
    2nd owner of Straight 8 SN1173
    5 other PDP-8's including an 8/i and a DECSet 8000
    SOL-20

  10. #120

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    Quote Originally Posted by DougIngraham View Post
    I can't think of why you would ever want to do that. In what context does that make sense? A 7500 is an SMA. Is this some kind of code obfuscation?
    Well, it was new to me, too. No idea why some conditionals are multiplied by 2, and others are multiplied by 4 (then incremented, IIRC?). Probably some slick memory saving hack we don't understand yet.

    is there a document describing 8BAL? I didn't see more than just the broad DECUS overview.

    (See how easily I am distracted?)
    There's the DECUS abstract as you have seen, but I haven't found a more extensive write-up or manual yet. I do see several mentions of dialects of FORTRAN where 8BAL was used to implement structured programming constructs in scientific applications. Unfortunately, no cool macro libraries seem to survive, either.

    Vince

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