Forum etiquette

Our mission ...

This forum is part of our mission to promote the preservation of vintage computers through education and outreach. (In real life we also run events and have a museum.) We encourage you to join us, participate, share your knowledge, and enjoy.

This forum has been around in this format for over 15 years. These rules and guidelines help us maintain a healthy and active community, and we moderate the forum to keep things on track. Please familiarize yourself with these rules and guidelines.

Remain civil and respectful

There are several hundred people who actively participate here. People come from all different backgrounds and will have different ways of seeing things. You will not agree with everything you read here. Back-and-forth discussions are fine but do not cross the line into rude or disrespectful behavior.

Conduct yourself as you would at any other place where people come together in person to discuss their hobby. If you wouldn't say something to somebody in person, then you probably should not be writing it here.

This should be obvious but, just in case: profanity, threats, slurs against any group (sexual, racial, gender, etc.) will not be tolerated.

Stay close to the original topic being discussed
  • If you are starting a new thread choose a reasonable sub-forum to start your thread. (If you choose incorrectly don't worry, we can fix that.)
  • If you are responding to a thread, stay on topic - the original poster was trying to achieve something. You can always start a new thread instead of potentially "hijacking" an existing thread.

Contribute something meaningful

To put things in engineering terms, we value a high signal to noise ratio. Coming here should not be a waste of time.
  • This is not a chat room. If you are taking less than 30 seconds to make a post then you are probably doing something wrong. A post should be on topic, clear, and contribute something meaningful to the discussion. If people read your posts and feel that their time as been wasted, they will stop reading your posts. Worse yet, they will stop visiting and we'll lose their experience and contributions.
  • Do not bump threads.
  • Do not "necro-post" unless you are following up to a specific person on a specific thread. And even then, that person may have moved on. Just start a new thread for your related topic.
  • Use the Private Message system for posts that are targeted at a specific person.

"PM Sent!" messages (or, how to use the Private Message system)

This forum has a private message feature that we want people to use for messages that are not of general interest to other members.

In short, if you are going to reply to a thread and that reply is targeted to a specific individual and not of interest to anybody else (either now or in the future) then send a private message instead.

Here are some obvious examples of when you should not reply to a thread and use the PM system instead:
  • "PM Sent!": Do not tell the rest of us that you sent a PM ... the forum software will tell the other person that they have a PM waiting.
  • "How much is shipping to ....": This is a very specific and directed question that is not of interest to anybody else.

Why do we have this policy? Sending a "PM Sent!" type message basically wastes everybody else's time by making them having to scroll past a post in a thread that looks to be updated, when the update is not meaningful. And the person you are sending the PM to will be notified by the forum software that they have a message waiting for them. Look up at the top near the right edge where it says 'Notifications' ... if you have a PM waiting, it will tell you there.

Copyright and other legal issues

We are here to discuss vintage computing, so discussing software, books, and other intellectual property that is on-topic is fine. We don't want people using these forums to discuss or enable copyright violations or other things that are against the law; whether you agree with the law or not is irrelevant. Do not use our resources for something that is legally or morally questionable.

Our discussions here generally fall under "fair use." Telling people how to pirate a software title is an example of something that is not allowable here.

Reporting problematic posts

If you see spam, a wildly off-topic post, or something abusive or illegal please report the thread by clicking on the "Report Post" icon. (It looks like an exclamation point in a triangle and it is available under every post.) This send a notification to all of the moderators, so somebody will see it and deal with it.

If you are unsure you may consider sending a private message to a moderator instead.

New user moderation

New users are directly moderated so that we can weed spammers out early. This means that for your first 10 posts you will have some delay before they are seen. We understand this can be disruptive to the flow of conversation and we try to keep up with our new user moderation duties to avoid undue inconvenience. Please do not make duplicate posts, extra posts to bump your post count, or ask the moderators to expedite this process; 10 moderated posts will go by quickly.

New users also have a smaller personal message inbox limit and are rate limited when sending PMs to other users.

Other suggestions
  • Use Google, books, or other definitive sources. There is a lot of information out there.
  • Don't make people guess at what you are trying to say; we are not mind readers. Be clear and concise.
  • Spelling and grammar are not rated, but they do make a post easier to read.
See more
See less

DB-25 serial port with 16 pins [Model 90]

  • Filter
  • Time
  • Show
Clear All
new posts

    DB-25 serial port with 16 pins [Model 90]

    While immersed in the hand to keyboard struggle to attach an IBM 6094-020 LPFK to a PS/2, I called on Michael Brutman to assist. He confirmed the bare minimum of lines needed, but he found a twist. Expect an update when he confirms it. Gist of the story - just because IBM published something only means that AT THAT TIME it was true... Maybe... the port info is "correct", but WRONG. Nothing horrible, but IBM figured that it didn't need all the wires.

    But anyways, while re-assembling my Model 90, I decided to look at the DB25 serial port. There are only > 15 < pins, all other positions are empty.

    The Model 90 was built from '89 to '94-ish. My hunch is that some older equipment in the mid 80s needed the extra lines. So, we have a DB25 serial [15 pins!] and a DE9.

    What serial equipment from the mid 80s used a DB25 serial port with 15 pins?

    IBM Model 90 DB25 Serial Port, 15 pins


    "x" pin
    "o" empty

    1 – No pin [IBM says N/C]
    2 Transmitted Data TD (“TXD”)
    3 Received Data RD (“RXD”)
    4 Request to Send RTS
    5 Clear to Send CTS
    6 DCE Ready—“Data Set Ready” DCR (“DSR”)
    7 Signal Ground—“Common Return”
    8 Received Line Signal Detector—“Carrier Detect,” “Data Carrier Detect” RLSD
    9 = +12 VDC reserved for testing
    10 = –12 VDC reserved for testing
    11 = Unassigned
    12 Data Signal Rate Selector (DCE Source)
    13 Secondary Clear to Send SCTS [IBM says N/C]
    14 – No pin [IBM says N/C]
    15 – No pin [IBM says N/C]
    16 – No pin [IBM says N/C]
    17 – No pin [IBM says N/C]
    18 – No pin [IBM says N/C]
    19 – No pin [IBM says N/C]
    20 DTE Ready—“Data Terminal Ready” DTR
    21 – No pin [IBM says N/C]
    22 Ring Indicator RI
    23 – No pin [IBM says N/C]
    24 Transmitter Signal Element Timing (DTE Source)—“External Clock” [IBM says N/C]
    25 – No pin [IBM says N/C]

    Note that many applications use only a subset of this pinout: 16 pins—1 through 8, 15, 17, and 20 through 25—for synchronous communication with secondary control and testing; 12 pins—1 through 8, 15, 17, 20, and 22—for ordinary sync applications; 7 pins—2 through 4, 6 through 8, and 20—for asynchronous communication with flow control; or the 4 pins 2, 3, 7, and 20 for bare-bones async applications that use software flow control.

    Do any of these devices ring a bell for an old style serial port?

    IBM PagePrinter 3812
    IBM LaserPrinter 4019-001
    IBM LaserPrinter 4019-E01
    Proprinter (R) 4201-001
    IBM Proprinter II 4201-002
    IBM Proprinter III 4201-003
    IBM Proprinter XL 4202-001
    IBM Proprinter II XL 4202-002
    IBM Proprinter III XL 4202-003
    IBM Proprinter X24 4207-001
    IBM Proprinter X24E 4207-002
    IBM Proprinter XL24 4208-001
    IBM Proprinter XL24E 4208-002
    IBM Personal PagePrinter 4216-010/D10/031
    Quietwriter (R) 5201-001
    IBM Quietwriter 5201-002
    IBM Quietwriter III 5202-001
    Quickwriter (R) 5204-001
    IBM PagePrinter 3816
    IBM ExecJet Printer 4072
    PS/1 (TM) Printer 2205 Model 001

    IBM 6180 Color Plotter
    IBM 6182 Auto Sheet Feed Plotter
    IBM 6184 Color Plotter
    IBM 6185 Color Plotter
    IBM 6186 Color Plotter Model 1,2
    IBM 7372 Color Plotter
    IBM 7374 Color Plotter
    IBM 7375 Color Plotter Model 1,2
    Attached Files
    Last edited by ardent-blue; June 16, 2021, 04:18 AM.

    AFAIK, the standard serial port on the PS/2 could not do synchronous protocol. There was a SDLC adapter for that, if I'm remembering correctly. Hence the unused pins, which are mostly associated with sync. (See here under V.24). I remember the Bell 208 and 209 modems. A blazing 9,600 bps over a leased line was the cat's whiskers.


      Most times all that is needed is pins 2, 3, 7 and either jumper 4 to 5 and 6 to 8 & 20. Or if hardware handshake is used then connect 4, 5, 6, 8 & 20 to handshake with the other device. But I guess you know that.
      Crazy old guy with a basement full of Pentium 1 laptops and parts


        I'll look at the reference.

        Had a flashback to common sums. 12 pins on upper pins, three pins on lower pins equals fifteen pins. Oops.

        I do know of the low conductor port, but everyone that designed this port is retired, dead, or both. IBM coulda sold [and did!] a DB25 to DE9 cable. So why they ran a DB25, partially pinned, and a smaller DE9, I dunno.

        Pin 1 is empty, Shield Ground
        Pin 20 DTE Ready
        Pin 22 Ring Indicator
        Pin 24 Transmit Signal Element Timing (DTE Source)

        ROLMphone 244PC 46900C 21A1482
        ROLMphone 120/DCM (Data Communication Module) 61300 84C9201
        ROLMphone 240/DCM (Data Communication Module) 62300A 79X0405
        ROLMphone 240E/DCM (Data Communication Module) 62380 21A1312
        ROLMphone 400/DCM (Data Communication Module) 64300A 79X0408
        ADCM (Asynchronous DCM) 42201 97D1108

        ROLMphone 244PC is mentioned on the net as providing data transfer over the serial port...

        Article Mentioned as having an RS--232 port.

        "Under the new regime, Centrex phones will be replaced with special units manufactured by
        IBM/Rolm, called "ROLMphones." These can be voice-only, or voice/data. The data option
        adds a 25-pin RS-232-C connector and accompanying electronics to the single-line or
        multiple-line voice telephone. Through a single pair of wires, a data-equipped phone can
        manage a voice conversation simultaneous with an asynchronous data connection up to
        19.2 Kbit/s, using a proprietary protocol known as "ROLMlink." This single instrument
        replaces the Centrex telephone, the LDS-125, and the user’s dialout modem."
        Last edited by ardent-blue; June 16, 2021, 04:43 AM.


          In my googling I found this page on the Schneider Euro PC. Maybe this can be the your standard?


            Pins 14-19 are empty.

            Originally posted by bolex View Post
            1. /INDEX
            2. /MOTORA (not for ext. drive!)
            3. /DSEL
            4. /DSELA (not for external drive!)
            5. /MOTOR
            6. /DIR
            7. /STEP
            8. /WDATA
            9. /WGATE
            10. /TRK0
            11. /WPROT
            12. /RDATA
            13. /SIDE1
            14-16. +5V DC
            17. NC
            18. PWRON
            19-25. GND


              The PS/2 used DC 37 connectors for its external floppy boxes. So not a floppy.


       says that the serial interface on the main board is type 3

                here is the manual that you should be looking at for the definitions of the type 3 serial interface on the model 90
                Last edited by Al Kossow; June 18, 2021, 04:55 PM.


                  Just a bog-standard async hookup. Not even with a CL option, like the original 5150 async card.


                    Originally posted by Al Kossow View Post
           says that the serial interface on the main board is type 3

                    here is the manual that you should be looking at for the definitions of the type 3 serial interface on the model 90
                    The Model 90 SSI seems to lack a serial port pinout.

                    HITR has Pin 24 as N/C. They call it "EIA-232-D" page 45 physical. The actual Model 90 DB25 serial port pinout differs in the presence of Pins 9-11 and Pin 24. Having said that, unless these pins are actually tested, they might be truly N/C from the planar... It depends...