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How to get output from SIO with Intel 8251A USART

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    #16
    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.

    Comment


      #17
      Oops, my mistake. Anyway, if I enter the program at 3700, still no success. I give up.

      Comment


        #18
        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.

        Comment


          #19
          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.

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            #20
            Unfortunately it doesn't have a pulse LED. I will look for one with this option.

            Comment


              #21
              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.

              Comment


                #22
                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.
                Frank
                Code:
                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.

                Comment


                  #23
                  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 "http://www.zmitac.aei.polsl.pl/ui/instrukcje/smiw7,9/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

                  Comment


                    #24
                    Dave, see the last paragraph of post #5 in this thread. It's been mentioned.
                    Reach me: vcfblackhole _at_ protonmail dot com.

                    Comment


                      #25
                      Chuck,

                      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?

                      Dave

                      Comment


                        #26
                        Agreed. It's probably not a bad idea to jumper 6,8 and 20.
                        Reach me: vcfblackhole _at_ protonmail dot com.

                        Comment


                          #27
                          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.

                          Dave

                          Comment


                            #28
                            UPDATE

                            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.

                            Comment


                              #29
                              Care to show your code? I think I might have a hunch.
                              Reach me: vcfblackhole _at_ protonmail dot com.

                              Comment


                                #30
                                Echo:

                                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

                                stream:

                                0000 db ff
                                0002 d3 03
                                0004 c3 00 00

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