Forum Rules and 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.

Rule 1: 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.

Rule 2: 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.

Rule 3: 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.

Rule 4: "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.

Rule 5: 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

How to get output from SIO with Intel 8251A USART

  • Filter
  • Time
  • Show
Clear All
new posts

    I don't understand. If you're toggling in the program at 0000, why are you branching to 3708?
    Reach me: vcfblackhole _at_ protonmail dot com.


      Oops, my mistake. Anyway, if I enter the program at 3700, still no success. I give up.


        Ok, program for starting at 0000:

        3E 4E
        D3 03
        3E 27
        D3 03
        DB 03
        E6 02
        CA 08 00
        DB 02
        D3 02
        C3 08 00

        Now, the INP indicator comes on, but the 8251 is again not enabled.

        The output of C8 goes not fully low, the "high" led on the probe is bright and the "low" led is a bit glowing.
        And at the CS pin on the 8251 same case.

        Why that? When I single step the program, the "high" led goes off and the "low" led goes on.
        Last edited by IBM_User; October 6, 2013, 11:30 AM.


          Remember that when you're running, the "chip enable" event is very short in comparison to all of the other stuff going on--so you will see a dim glow on the "LOW" LED. That's normal. I assume that your logic probe doesn't have a pulse detector (i.e. a separate LED that blinks in response to a pulse of any length). That makes things a bit harder, it that's the case.
          Reach me: vcfblackhole _at_ protonmail dot com.


            Unfortunately it doesn't have a pulse LED. I will look for one with this option.


              Here's a pretty decent one--note that it has 3 LEDs, with the third one being able to "memorize" a pulse or simply lengthen it. The old Radio Shack "Micronta" probe was perhaps the most popular, but I haven't seen a new one in years. There are Chinese clones, however.
              Reach me: vcfblackhole _at_ protonmail dot com.


                The 8251 is a bit tricky to initialize. (Vary from different manufacturers)
                Important is, that you initialalize the 8251 quickly after the reset (Hardware-Reset).
                The time between the OPs for the initializing must be short.
                Send about three 00H befor going on.
                The code underneath works for me well (For two 8251s).
                Have a try.
                CCOM	.EQU    0B1H            ;8251 COMMAND PORT.
                RCOM	.EQU    0F1H	     ;CONSOLE COMMAND PORT.
                	MVI  	A,00H		;INITIALIZE UART.
                	OUT  	CCOM
                	OUT	RCOM
                	MVI  	A,00H		;INITIALIZE UART.
                	OUT  	CCOM
                	OUT	RCOM
                	MVI  	A,00H		;INITIALIZE UART.
                	OUT  	CCOM
                	OUT	RCOM
                	MVI  	A,40H		;reset command
                	OUT  	CCOM
                	OUT	RCOM
                	MVI  	A,0CEH		;Mode register 8,2,n /16/64 (CE=9600; CF=2400)
                	OUT  	CCOM
                	MVI  	A,0CFH		;Mode register 8,2,n /16/64 (CE=9600; CF=2400)
                	OUT  	RCOM
                        MVI  	A,37H		;Command register
                	OUT  	CCOM
                	OUT	RCOM
                Last edited by Frank S; October 6, 2013, 10:38 PM.


                  Hi again.

                  Right, I am happy with your program. Keep using that every time!

                  You software appears to initialise the 8251 in Asynchronous, 8N1 x16 clock mode, wait for a character to be received and echo it back to whoever sent it - repeat for infinity. If this is loaded at zero and executed I would expect it to work (with one exception - that I will discuss in a second).

                  I agree with Chuck. If you stick an LED on any (active low) signal line to monitor it - it will normally be ON (assuming HIGH = logical '1' = LED ON) and will only pulse low very briefly.

                  Your transmitter will not, however, still transmit anything!

                  Check out the 8251 data sheet ",9/INT8251.pdf" on page 4 under "Transmit Buffer". In order to transmit anything, the 8251 needs to see /CTS low. You said in a previous post that you have only wired up pins 2, 3 and 7 (Tx, Rx and GND). What have you done with /CTS? The usual 'trick' is to wire /RTS to /CTS and /DTR to /DSR. This 'fools' the 8251 into thinking that everything is OK and it will quite happily transmit and receive characters.

                  Well done for sticking with it though!



                    Dave, see the last paragraph of post #5 in this thread. It's been mentioned.
                    Reach me: vcfblackhole _at_ protonmail dot com.



                      And I mentioned it back in post #10 as well - but that was a while ago and the OP may have forgotten by now.

                      As far as I can see (barring hardware faults) the software should now work - the OP should (hopefully) just need to jumper /RTS and /CTS to make their hardware set-up work; so why not mention it here again to make life simple?



                        Agreed. It's probably not a bad idea to jumper 6,8 and 20.
                        Reach me: vcfblackhole _at_ protonmail dot com.


                          On a 25-way D connector, link 4-5 (RTS-CTS) and 6-8-20 (DSR-CD-DTR). Pins 2, 3 and 7 are then the data lines (2 and 3) and ground (7). The 'sense' of the signals may change depending upon whether the end is "Data Terminal Equipment" (DTE) or "Data Communication Equipment" (DCE).

                          To be safe, undertake the links on both ends of the cable just to make sure that both the terminal and the SIO UART are happy.




                            I got an another IMSAI SIO board, now I'm able to echo characters from the keyboard back to the screen.
                            For this I use the ECHO program from the manual.

                            Unfortunately I can't get a continuous stream of characters on the screen, and I dont know why because
                            the ECHO program works just fine.


                              Care to show your code? I think I might have a hunch.
                              Reach me: vcfblackhole _at_ protonmail dot com.



                                0000 3e 4e
                                0002 d3 03
                                0004 3e 37
                                0006 d3 03
                                0008 db 03
                                000a e6 02
                                000c ca 08 00
                                000f db 02
                                0011 d3 02
                                0013 c3 08 00


                                0000 db ff
                                0002 d3 03
                                0004 c3 00 00