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Thread: Question about 3270 Coax Protocols

  1. #1

    Default Question about 3270 Coax Protocols

    Hi all,
    I've been trying to learn about the protocol IBM used between their coax terminals and the controller such as the 3174. I've found some documentation on the Type B terminals like the 3277, but I'm more interested in the Type A implementation, used for the 3278/3279. I'm not looking for a detailed explanation of the 3270 protocol since I know it can fill multiple books, but rather just hoping someone can point me in the right direction.
    Pretty much everything I know at the moment comes from the CHIPS 82C570 data sheet. Given the amount of ISA emulation boards and clone terminals made I would think this information must be out there somewhere.

  2. #2

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    There is a wealth of information in this bitsavers.org directory:

    http://www.bitsavers.org/pdf/ibm/3270/

    This document gives a good, basic and concise description of the basic 3270 protocol over coax, including the electrical and timing specs and the basic 13-bit 3270 frame:

    http://www.bitsavers.org/pdf/ibm/327...Spec_May74.pdf

  3. #3

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    Thanks for the links, although from what I've gathered the documents there only describe the connection to a 3277 terminal, which is of Type B (or is it Category B).

    Well, my other option is to get a 3174 running, but as people have pointed out the microcode is hard to find and harder to duplicate. But I was wondering if it would be easier to image and emulate the 20mb fixed disk drive. It appears to be a ST506/412 drive in a specialized caddy. Has anyone tried this?

  4. #4

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    Just curious about what you are wanting to achieve. Are you wanting to connect a 3278/3279 to a modern system and use it as some sort of console?

    BTW, from what I understand the disk drive in the 3174 was just a standard MFM disk drive of that era. Someone with such a working drive from a 3174 should be able to image it for you if they are willing.

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    Quote Originally Posted by cfsimmons View Post
    Thanks for the links, although from what I've gathered the documents there only describe the connection to a 3277 terminal, which is of Type B (or is it Category B).

    Well, my other option is to get a 3174 running, but as people have pointed out the microcode is hard to find and harder to duplicate. But I was wondering if it would be easier to image and emulate the 20mb fixed disk drive. It appears to be a ST506/412 drive in a specialized caddy. Has anyone tried this?
    The microcode is easy to duplicate, but the 2.44 floppy disks can be hard to find. Some one else has used the project here http://www.pdp8online.com/mfm/ to replace the disk.

    Finding a working drive is hard. I have three, but none work reliably. I think they suffer if not properly parked.
    Dave
    G4UGM

    Looking for Analog Computers, Drum Plotters, and Graphics Terminals

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    Quote Originally Posted by MossyRock View Post
    Just curious about what you are wanting to achieve. Are you wanting to connect a 3278/3279 to a modern system and use it as some sort of console?

    BTW, from what I understand the disk drive in the 3174 was just a standard MFM disk drive of that era. Someone with such a working drive from a 3174 should be able to image it for you if they are willing.
    The 3174 will talk TN3270 so can connect to Hercules. I have used it that way
    Dave
    G4UGM

    Looking for Analog Computers, Drum Plotters, and Graphics Terminals

  7. #7

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    Quote Originally Posted by MossyRock View Post
    Just curious about what you are wanting to achieve.
    Ultimately, I'd like to be able to talk to these terminals. Connecting one to Hercules is an end goal. The only terminal I have at the moment is a 3270 PC, but I'd love to get a 3279. I won't bother though until I know i can get it working.
    I'm glad to hear that someone has emulated the drive, although the problem of getting the microcode still remains.

    My knowledge of mfm drives is pretty limited. Would a typical pc controller be able to read the raw data from the drive, or can the 3174's disk controller be implemented differently?

  8. #8
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    Quote Originally Posted by cfsimmons View Post
    I'm glad to hear that someone has emulated the drive, although the problem of getting the microcode still remains.
    Is much known about the Memorex/Telex 1174, like
    https://www.ebay.com/itm/191672782519 ?

    I bought one a while ago because it used SCSI drives and commodity ICs.

    Haven't had a chance to do much more than image the drive from it.

  9. #9
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    Quote Originally Posted by Al Kossow View Post
    Is much known about the Memorex/Telex 1174, like
    https://www.ebay.com/itm/191672782519 ?

    I bought one a while ago because it used SCSI drives and commodity ICs.

    Haven't had a chance to do much more than image the drive from it.
    Al,
    I did have a couple of 3.5" floppy disks from a memorex telex controller but no idea if they are any use. I will have a try at imaging them if I can find them
    Dave
    Dave
    G4UGM

    Looking for Analog Computers, Drum Plotters, and Graphics Terminals

  10. #10

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    Quote Originally Posted by cfsimmons View Post
    Ultimately, I'd like to be able to talk to these terminals. Connecting one to Hercules is an end goal. The only terminal I have at the moment is a 3270 PC, but I'd love to get a 3279. I won't bother though until I know i can get it working.
    I'm glad to hear that someone has emulated the drive, although the problem of getting the microcode still remains.

    My knowledge of mfm drives is pretty limited. Would a typical pc controller be able to read the raw data from the drive, or can the 3174's disk controller be implemented differently?
    Your end goal is what I was trying to do, but I was going to use an actual 3174. Things were going well until I bought an IBM 3278 on eBay but discovered that it had no keyboard when it arrived. Long story short, my mistake. Now I can't find a keyboard anywhere for less than $2,000, and I'm not going to take out a loan for this.

    Unless you have good contacts in the vintage computing community, finding a working, complete 3279 is going to be very difficult (and probably costly).

    The 3270 protocol is a insanely complicated beast. I can't imagine trying to emulate it from scratch unless you are an electrical engineer AND a low-level programming whiz. Unless I'm misunderstanding something here, just emulating a MFM drive from a 3174 is only a fraction of what is required as you have none of the required infrastructure hardware for a 327x terminal to work. To go the normal route, you're going to need something like an IBM 3174-23L (with the ethernet option) to be able to telnet into Hercules' console port to get the console displayed on a coax terminal like the 3279. If the 3174 has an option for a hard disk, THEN you can install the MFM emulator in it with the appropriate IBM microcode (I think it is Configuration Support 'C' version 6.2 or higher).

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