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Build your own PDP 8I, Part 2..

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  • daver2
    replied
    Yes, I have the second edition of the book.

    I was trying to answer DDS about why the Lab notes talk about a 7400 as an AND gate in some places.

    Yes, you really need to read the Lab notes in conjunction with the book (since the lecture notes are not available to us and the book was - I think - recommended reading for the course).

    Back in post #117 you were asking "I could use an Explanation from Daver2 on A1 thru A9 and on F0 thru F10, to clarify things some..". This was what I was meaning to do. Do you now understand what the Fx and Ax signals do (hence I don't need to provide the explanation any more)?

    I don't see why you are going to all the trouble of equalising the gate delays to such things as the ALU? From the register outputs through the MUX and the ALU back to the registers is a combinatorial path (i.e. devoid of clock signals). All that matters is that the data arriving back at the registers is stable just before the clock latches the data back into the registers. It may help if noise is present - but that is more by luck than design.

    Just as a matter of interest - are you using Fairchild 9024 JK flip-flops in your implementation or have you replaced these with an alternative?

    Dave

    Leave a comment:


  • Marty
    replied
    Hi All;

    Dave, Thanks for the explanation..
    I don't know if You have either book, But, He is trying to show/teach mixed logic, that way His students would get used to seeing either symbol for a the type of logic that is being used to convey, what is happening..
    So, I think either book most likely do a better job of conveying this, or maybe after writing this numerous times the explanation just got better..

    "" Sorry - work is getting in the way of answering your Fx/Ax query. I will get around to it shortly - I promise. ""

    I didn't know I had a query.. I was just stating what I was doing..
    Such as, in LD-13, ALU Signals, the schematic for the Cin.. Shows the signals to M10 (7430), that they arrive at the 7430 at different times, so what I am trying to do is make them all, closer in time together..
    On the 7430 pin 1,2 You have a four gate delay, on pin 3, it is the same thing, pin 4 and pin 5 have a one gate delay..
    And for the A0, F2 and F7 a No gate delay.. So I redid the circuit, so that they all have a two gate delay and so all the signals arrive at the 7430 at the same time.. And the Output of the 7430 goes to another gate, thereby the total is a three gate delay, which is the same as what is used for the S0, S1, S2, S3 and M signals as well.. That way all of these signals arrive at the same time to the ALU..

    THANK YOU Marty

    Leave a comment:


  • daver2
    replied
    "A 74xx00 is not an AND gate. It's a NAND gate. And it's called a NAND gate in the Texas Instruments catalog that the instructor refers to." I think that this is a Typo..

    Well, not really...

    The Lab manual on page 15 identifies a 7400 as an "AND" gate (even though it is a NAND gate) and also a 7402 as an AND gate (even though it is a NOR gate). Then look down to page 31 of the Lab manual and you will see that a 7400 is described as an OR gate! What's going on?

    It depends upon whether you are drawing (what I will call) a "physical" schematic or a "logical" schematic. If you were drawing a physical schematic you would draw each gate of a 7400 as a NAND gate - end of story. If you were drawing a logical schematic you could draw a 7400 gate as either a NAND gate or as an OR gate with inverted inputs.

    Check out the first schematic of "LD8 - LD23 Schematics.pdf" in the upper right hand corner (if printed the right way around in landscape that is). You will see a 7430 (E15/A21) which is an 8-input NAND gate drawn as an OR gate with inverted inputs. Look back a bit to G4/A49 (a 7400) - this is a NAND gate package drawn as an OR gate with inverted inputs. Even more bizarre is K8/A28 and K7/A25 (both 7402 NOR packages). One is drawn as a NOR gate 'properly' with the other drawn as an AND gate with inverted inputs.

    This indicates that the schematics are drawn logically rather than physically - so you need to beware of this when reading them.

    Just in case you are confused at this point, DeMorgan's states that a NAND gate can be replaced by an OR gate with inverted inputs and a NOR gate can also be replaced by an AND gate with inverted inputs. (i.e. you can physically create a NOR gate with an AND gate and some inverters - or you can draw a NOR gate as an AND gate with inverted inputs).

    You must also remember that this is a Lab associated with a University Course - so some of the material may not follow what you expect from (say) an electronics constructional project. Whilst reading the book that Marty recommended, I came across a line symbol on some wires that I did not understand. I had to go back to earlier chapters of the book to understand what this particular symbol actually meant in the context of the course. I have also seen this same symbol numerous times on the Lab schematics - so understanding why it is there is important.

    Sorry - work is getting in the way of answering your Fx/Ax query. I will get around to it shortly - I promise.

    Dave

    Leave a comment:


  • Marty
    replied
    Hi All;

    DDS, Thank You for the Response..
    "" A 74xx00 is not an AND gate. It's a NAND gate. And it's called a NAND gate in the Texas Instruments catalog that the instructor refers to. ""
    I think that this is a Typo..
    "" I'm going to bet there was a lot of additional explanation going on in the lecture sessions. ""
    Yes, I wish I had a Recording of those lectures..
    "" There's also a tricky part about the clock design and whether or not the clock should be "gated" that you might want to look at. He mentions that the design could be more fault tolerant if a chip that wasn't then readily available were used. ""
    He used a 555 for the clock chip and buffered it with numerous NAND and NOT gates..
    "" He also mentions the use of a 74Lxx chip in one spot because they're less likely to trigger on spikes. ""
    Also the 74L00 was used as it has the longest Delay Time, for Lengthening the flip-flop pulse..
    "" When you mentioned earlier that you were getting different results on different test runs I started thinking "unstable clock". Might be a good place to look. ""
    As mentioned Earlier, I used a very slow clock pulse, But my pulse was half cycle, same time up as time down.. Which also might have been some of the problem, Instead of like ten to one.. One time up and nine times down for settling..

    THANK YOU Marty
    Last edited by Marty; September 24, 2015, 04:13 AM.

    Leave a comment:


  • DDS
    replied
    I got some slack time today and (mis)used some of it to slog through parts of the Lab Manual. All I can say is it's clear as mud in some places. Even when I was pretty sure I knew what he was trying to say I was still shaking my head at the end of his explanation. I would have been one very unhappy camper if i was in his class and the Lab Manual was all I had to go on. I'm going to bet there was a lot of additional explanation going on in the lecture sessions. Just one small example. A 74xx00 is not an AND gate. It's a NAND gate. And it's called a NAND gate in the Texas Instruments catalog that the instructor refers to. There's also a tricky part about the clock design and whether or not the clock should be "gated" that you might want to look at. He mentions that the design could be more fault tolerant if a chip that wasn't then readily available were used. He also mentions the use of a 74Lxx chip in one spot because they're less likely to trigger on spikes. When you mentioned earlier that you were getting different results on different test runs I started thinking "unstable clock". Might be a good place to look.

    Leave a comment:


  • Marty
    replied
    Hi All;

    Dave, Thank You for the compliment ..

    "" I give you 10 out of 10 for your notebook - very neat and tidy! ""
    I don't know of any other way, to keep everything together and in a somewhat organized manner..
    Where any changes are easy to make, and figure out..

    I have added Timing delays to some of Fxx and Axx signals, where needed, so that the signals come in at the same time, as the rest of the circuit.. Not Elegant, but it should serve the purpose..

    For present, I think I have it all drawn out.. I need to put in Gate numbers and pin numbers next..
    I am also making a WireList for this unit, with signal names..

    THANK YOU Marty
    Last edited by Marty; September 23, 2015, 05:01 PM.

    Leave a comment:


  • daver2
    replied
    Marty,

    I give you 10 out of 10 for your notebook - very neat and tidy!

    Dave

    Leave a comment:


  • Marty
    replied
    Hi All;

    Dave, Thank You for the file, and the information.. I was hoping to eliminate some gates..

    I have almost copied most of the New Schematics and then I will need to assign gate numbers and pin assignments..
    And see if there are a few less Ic's and a more smoother timing situation..

    THANK YOU Marty

    Leave a comment:


  • daver2
    replied
    The attached PDF file I produced from an Excel spreadsheet identifies that your proposed 4-input NAND gate substitution will not give the same result as the three separate 2-input NAND gates.

    I have identified the two inputs to G2 pins 4 and 5 as A and B and the output from pin 6 as Q1 respectively.

    The output from G2 pin 8 as Q2.

    And the output from K5 pin 11 as Q3.

    I also computed the output from your transformation as "BIG NAND".

    It can be seen that the columns headed Q3 and "BIG NAND" are not identical - hence your proposed transformation will not work.

    For example, I used the Excel expression "=NOT( AND(A2,B2))" in the individual cells to calculate Q1.

    You can use the Excel method as a quick and dirty test for any boolean expression.

    When more than one FALSE started to come out of the calculation of Q3 - this indicated that a simple gate was not sufficient support the simplified logic. A NAND gate will only produce a single FALSE output when all of its inputs are TRUE (or some of them false if you use a NOT gate between them as your proposal with DCA). A single FALSE output from a simple NAND gate can, therefore, never replicate a logic system with more than one FALSE output.

    I knew about the 2102 SRAM chips themselves - but good thinking about the logic to control the extra RAM chips and the serial interface.

    Dave
    Attached Files
    Last edited by daver2; September 23, 2015, 09:20 AM.

    Leave a comment:


  • Marty
    replied
    Hi All;

    Daver, Thank You for Your Response, and answers..

    "" I have been comparing various sources of the list of ICs tonight and most of the sources agree - but one thing that jumps out to me is that there are more logic chips on the photograph of the wire-wrapped board than are in the kit.

    One possibility that could account for the difference is that the board in the photograph has the memory expanded to a full 4K. I have been ignoring the memory devices themselves - but I am wondering if there are any additional 74XX series logic accompanying the memory expansion. I will check this out later. ""

    Yes, the Extra Ic's are for Both the Memory and the I/O Serial Port..
    It uses 48 each 2102's just for the memory, along with other support Ic's, and another bunch for the I/O..

    "" I have also been identifying 'issues' on the schematic diagrams (e.g. unconnected input pins such as clocks, /preset, J and K inputs etc.). When I have checked the wiring lists though - they are correctly connected. The outcome is that if you are following the schematics for wiring then you will be mislead (i.e. some critical inputs will be left floating - and hence little antennas for picking up noise). If you are following the wiring lists themselves - you should be OK. ""

    I wired from the ExtSorted List, and not the Schematic..
    There are things that are Not in the Schematic, that are in the WireList.. Like all of the Muxes, what is in the Schematic, is only a Partial Schematic of the Mux circuitry, along with the Registers..

    "" handling INDIRECT operands and the like - or rather generating the sequence for such processing. ""
    That is what I need the most at the present..

    "" I will write up a description of it later when I am not too tired (it is pretty late in the UK now). "" That's fine..
    "" I won't answer your NAND gate query above at this late hour - I will only screw up the Karnaugh Map! "" That's fine..

    THANK YOU Marty
    Last edited by Marty; September 24, 2015, 04:05 AM.

    Leave a comment:


  • daver2
    replied
    Just read a bit more of post #117.

    The F and E in the book stands for Fetch and Execute. The numbers after them just refer to specific Fetch and Execute States.

    These should not be confused with the F and A states in the LD12 - these are all for the Fetch state!

    Let me put together a more considered description of the LD12's Fetch sequencer.

    Dave

    Leave a comment:


  • daver2
    replied
    I have been comparing various sources of the list of ICs tonight and most of the sources agree - but one thing that jumps out to me is that there are more logic chips on the photograph of the wire-wrapped board than are in the kit.

    One possibility that could account for the difference is that the board in the photograph has the memory expanded to a full 4K. I have been ignoring the memory devices themselves - but I am wondering if there are any additional 74XX series logic accompanying the memory expansion. I will check this out later.

    I have also been identifying 'issues' on the schematic diagrams (e.g. unconnected input pins such as clocks, /preset, J and K inputs etc.). When I have checked the wiring lists though - they are correctly connected. The outcome is that if you are following the schematics for wiring then you will be mislead (i.e. some critical inputs will be left floating - and hence little antennas for picking up noise). If you are following the wiring lists themselves - you should be OK.

    Yes, the F and A logic is the heart of the "fetch sequencer" and understanding of this is paramount to knowing how an instruction is fetched and processed before being executed. It is this logic that is responsible for detecting an interrupt, fetching the opcode from memory to the IR, incrementing the PC, handling INDIRECT operands and the like - or rather generating the sequence for such processing. I will write up a description of it later when I am not too tired (it is pretty late in the UK now).

    I won't answer your NAND gate query above at this late hour - I will only screw up the Karnaugh Map!

    Dave

    Leave a comment:


  • Marty
    replied
    Hi All;

    I have a question, about a sequence of gates in the accompanying pictures..

    Nand Gate (G2 ) pins 4, 5 and 6 connects to Nand gate (G2 ) pins 9, 10 and 8 and this connects to Nand gate (K5 ) pins 12, 13 and 11..

    Can I connect these to a 7420 4 input Nand gate with signal DCA Inverted, So that instead of having three Nand gates, I have one Nand gate ??

    001.jpg 002.jpg

    THANK YOU Marty

    Leave a comment:


  • Marty
    replied
    Hi All;

    DDS, Thank You for the information..
    "" I don't have either edition of the book on the LD20 and LD30 and have not spent enough time looking at the documentation you posted on the LD12, ""
    Actually, what is in either Book is Quite different than what is in the Manual for THIS Design..
    You need to use this Manual for the Descriptions of these signals, though it might be somewhat incomplete..

    "" but I'd like to point out that designers often give meaningful names to components of the design even if they look like gibberish to others. ""
    Actually, they were pretty good at giving names to the different signals..

    "" For Example, #1ESS/#1AESS peripheral scanners often have a lead involved with the enable/enable verify sequence called "WRMI" (pronounced wormy) that stands for "We Really Mean It." And in the #1AESS Tape Unit Controller (TUC) schematic, the gate names of the input buffer shown in a long line across the bottom of one sheet spell out "THE TUC NEEDS ALL THE HELP IT CAN GET". ""
    I would like to know more about the #1ESS.. Do You know anything about Step by Step Systems ??

    "" So I'm wondering if the names A0 through A9 and F0 through F10 have any description of their names or functions in the two editions of the book. Not Really.. But, there is a Description in the Manual that is with these files..
    They do have an F-series and and E-series in the books, which You could read and compare with what is here..

    I could use an Explanation from Daver2 on A1 thru A9 and on F0 thru F10, to clarify things some..

    I have re-drawn the LD-8 thru LD-14, I have LD-15 thru LD-21 to go, in my next notebook..

    THANK YOU Marty

    Leave a comment:


  • DDS
    replied
    "I had Read and copied most of the Manual, but, it didn't make sense to me.. So, I don't fully understand A0 thru A9 and F0 thru F10 and their function, and what is used for what and where and why.."

    I don't have either edition of the book on the LD20 and LD30 and have not spent enough time looking at the documentation you posted on the LD12, but I'd like to point out that designers often give meaningful names to components of the design even if they look like gibberish to others. For Example, #1ESS/#1AESS peripheral scanners often have a lead involved with the enable/enable verify sequence called "WRMI" (pronounced wormy) that stands for "We Really Mean It." And in the #1AESS Tape Unit Controller (TUC) schematic, the gate names of the input buffer shown in a long line across the bottom of one sheet spell out "THE TUC NEEDS ALL THE HELP IT CAN GET". So I'm wondering if the names A0 through A9 and F0 through F10 have any description of their names or functions in the two editions of the book.

    Leave a comment:

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