            UM82C11-C Bidirectional Printer Port Modification

            =================================================



This is a bidirectional modification for computers using the 82C11 for their

parallel ports.  Please note that we give no guarantee that it will work or

that everything is correct!  Proceed at your own risk!  (There is only one

mistake as far as we know, but everybdy knows how easy it is to make a typo.)



NOTE: An asterix (*) in front of a control signal indicates a negative true

signal (low = 1).  eg. CS=true when high, *CS=true when low.



The UM82C11-C is almost a one chip IBM pc printer interface.  It includes

all the the registers and buffers for a printer port and needs only some

address decoding and a LS245 buffer to hook a PC to a printer (plus some wire

of course).  It comes in two packages - a 40 pin dual in line (DIP) package,

and a 44 lead flat pak.  The pin numbers referred to below are for the DIP

package.  There is a pinout for both packages at the end of this article.



Pin 19 on this chip is the Printer Data Output Enable (*POE).  The printer

data buffers drive the printer data bus when this pin is grounded.  On my 

machine, and probably many others, this pin is simply connected to pin 20 

(Ground) next to it to permanently enable the printer data bus.  This pin

needs to be controlled by the D5 output of the printer control word to make 

the port bidirectional.  Unfortunately the printer control word is internal 

to this chip and we can't simply add a couple of jumpers (as suggested in 

Steve Ciarcia's article in Byte-Sept/88, pp 304-305,310)



On examining the truth table and some timing diagrams in the spec sheets we

find that when *CS and DIR are low and A1 is high we can use the rising edge 

of *IOW to latch the printer control word.  After detailed analysis and 

consideration of our junkbox the following design presented itself.  We used

a 74LS10 triple 3 input NAND gate as a decoder and a 74LS379 Quad D flip-flop

with Clock enable to latch the data.



                            Schematic Diagram

                            =================

                        74LS10                       74LS379



                 ------\

  (8)DIR --------|1     \                           D5(14)   /-------tie

               /-|2    12>0---\                      |       |  |  |

(40)+5v--\/\/\-+-|13    /     |                ------------------------

          4k7  | ------/      |                |     4 (D0)  5  12 13 |

               | ------\      |    (6) *IOW ---|9 (CP)      (D1,D2,D3)|

       tie ----+-|3     \     |                |                      |

               \-|4     6>0-\ |             /--|1(*CE)                |

 (18)*CS --------|5     /   | | ------\     |  |     2 (Q0)           |

                 ------/    | \-|11    \    |  ------------------------

                            \---|9     8>0--/        |

 (39) A1 -----------------------|10    /           *POE (19)

                                ------/

 (40) +5 --(LS10-14, LS379-16)    A 0.01uf decoupling capacitor was added

 (20)GND --(LS10--7, LS370--8)      between +5 and GND.

     (The numbers in paranthesis are the pin numbers on the 82C11.)



                          Wiring Diagram

                          ==============



       40              39              40                           6

       +5              A1              +5                          *IOW

        |       /---\   |   /-\ /---\   |                           |

        |  tie  |   |   |   | | |   |   |          tie tie          |

      ------------------------|---  | --------------------------------

      |14  13  12  11  10   9 | 8|  | |16  15  14  13  12  11  10   9|

      |+5                     |  |  | |                              |

      |                       |  |  | |                              |

      |1    2   3   4   5   6 | 7|  | | 1   2   3   4   5   6   7   8|

      ------------------------|---  | --------------------------------

       |   tie tie tie  |   | | |   |   |   |       |  tie          |

       |                |   \-/ |   \---/   |       |               |

      DIR              *CS     GND         *POE     D5             GND

       18               18      20          19      14              20



 (40) +5 --/\/\/--tie  (All pins marked tie are pulled up to +5v through

            4k7                    this resister. (4.7k ohms))



 (40) +5 ---||---GND(20) (This capacitor is connected across the supply

            10n                  for decoupling. (.01 uf))





I built this circuit on a small piece of perf board and hot melt glued it on

top of the 82C11.  The wires were kept as short as possible and tack soldered

to the pins of the 82C11.  The circuit board trace connecting pin 19 to GND

(pin 20) was cut.  The printer seems to work fine and the CHECKDDT program

reports that my printer port is now bidirectional.  I have yet to finish

constructing the DDT51 to see if it really is working as it needs to, but I

don't see any reason why it shouldn't be.



The next challenge is to see if this can be reduced to one IC.  A PAL or 

something similar should be easy to program for this.





          -------------------------------------------------------



                    Alternate Schematic Diagram (untried)

                    =====================================



  This should do the same thing, and the parts might be more common.



                                            ----------

 (14) D5 -----------------------------------|2      5|---------*POE (19)

                                            |        |

                 ------\   -------------    | 74LS74 |

  (8)DIR -------O|4     \  |         15|O---|3       |

 (18)*CS -------O|5      >O|           |    |  1  4  |

 (39) A1 --------|6     /  | 74LS138   |    ----------

                 ------/   |           |       |  |

  (6)*IOW -----------------|1          |       \--+--/\/\/\---+5v

                       /---|2          |              4k7

                GND ---+---|3          |

                           -------------



 (40) +5 --(LS14-14, LS138-16)

 (20)GND --(LS14--7, LS138--8)

     (The numbers in paranthesis are the pin numbers on the 82C11.)







          -------------------------------------------------------



                            Data on the 82C11

                            =================



                 UM82C11-C pinout

                 ================



     This is the 40 pin Dual in Line package.



                    ___________

Crystal in       x1 | 1     40|  Vdd   Power Supply +5v

Crystal in       x2 | 2     39|  A1    System Address 1

Clock out       CLK | 3     38|  A0    System Address 0

Clock / 10     DCLK | 4  U  37|  P0    Printer Data Bus 0

Reset           RST | 5  M  36|  P1    |

I/O Write      *IOW | 6  8  35|  P2    |-Data to Printer

I/O Read       *IOR | 7  2  34|  P3    |

Direction       DIR | 8  C  33|  P4    |

System Data Bus  D0 | 9  1  32|  P5    |

              |  D1 |10  1  31|  P6    |

              |  D2 |11  -  30|  P7    Printer Data Bus 7

              |  D3 |12  C  29|  *ERROR  Error input from printer

              |  D4 |13     28|  SLCT    Select input from printer

              |  D5 |14     27|  PE      Paper end input from printer

              |  D6 |15     26|  *ACK    Acknowledge input from printer

System Data Bus  D7 |16     25|  BUSY    Busy input from Printer

Interrupt       IRQ |17     24|  *STROB  Strobe output to Printer

Chip Select     *CS |18     23|  *AUTOFD Printer feeds one line after printng

Data output En *POE |19     22|  *INIT   Initialize printer

Ground (0v)     Vss |20     21|  *SLCT   Select Printer

                    -----------



   There is also a flat pack 48 pin version of this IC.  Pin 1 on the flat

   pack is *IOW and the pins follow the same sequence around the package with

   four not connected pins (pins 40-43) between VDD and X1.



                  P0           *SLCT

                  36            25

                 ------------------

          A0   37|                |24 PE

                 |                |

   (4 NC lines   |                |

    pins 40-43)  |                |

                 |                |

        RST    48|                |13 D7

                 ----------------/

                  1               12

                  *IOW            D6





The data sheets give the following truth table for the operation of this chip:



            Input            Output Operation:

  *CS   A1  A0  *IOR   *IOW    DIR

    1    x   x    x      x      0    Inactive    (x=don't care)

1)  0    0   0    1      0      0    Write Data to Printer

2)  0    0   0    0      1      1    Read data on printer Bus

3)  0    0   1    0      1      1    Read Status from Printer

4)  0    1   0    1      0      0    Write Control word to the printer

5)  0    1   0    0      1      1    Read control word on Printer control bus

    0    1   1    0      x      x    *****Illegal operation*****

    0    ---Others---------------    No Action



The 82C11 supports 5 commands: (Addresses are for LPT1).



1)  Write data to printer (Out &H378,data)  The data written to the 82C11 is

latched into the output register.  *POE enables or disables the data from

being driven onto the printer data bus (GND=Enabled).



2)  Read data from the printer bus (data = in &h378).  The data on the printer

data bus is read back to the computer.  If *POE is enabled(GND) this will

reflect the output data.  If *POE is disabled (+5) it will reflect whatever is

on the other end of the printer data bus.



3)  Read Status from the Printer (data = in &h379).  The data reflects the

status of the printer status lines.



        D7 = Inverted version of BUSY signal - When this goes high the

                        printer is ready to accept data.

        D6 = *ACK  - The printer pulls ACKnowledge low to indicate it has

                        received the data and is ready for more.

        D5 = PE    - Paper End - A high indicate the printer is out of paper.

        D4 = SLCT  - SeLeCT -This signal is high when the printer is on-line.

        D3 = *ERROR  - This pin indicates the printer has a problem.

        D2 = Not used

        D1 = Not used

        D0 = Not used



4)  Write Control word to the printer (out &h37a,data).  Send control signals

to the printer.



        D7 = Not used

        D6 = Not used

        D5 = Not used     ***(we use this to enable/disable bidirectional)***

        D4 = IRQEN - When this pin is high the 82C11 sends an Interrupt

                        request to PC when *ACK is asserted by the printer.

        D3 = SLCT      - ??

        D2 = INIT      - INITiate printer (reset printer)

        D1 = *AUTOFD   - When low the printer automatically feeds one line

                                after printing.

        D0 = STROB     - This pin is set to a 1 to tell the printer to read 

                                the data on the printer data bus.



        Pins D3, D2 and D0 are inverted between the control word and the 

        output pins that connect to the printer. (eg. A 1 on D0 will be 

        actually sent to the printer as a low on the *STROB pin)



5)  Read Control word from the printer (data=in &h37a).  Read what control

signals have been sent to the printer.  Data bits are the same as above.

Note that this will not read what has been stored on D5.



       ----------------------------------------------------------



February 27, 1991



I welcome any constructive criticism.  I know that I do not know everything

there is to know about the parallel port and I may have made some mistakes.

Please let me know so I can fix them.



Thank you.



David Meed           (506) 375-4029

c/o NBBI

Victoria, NB

E0J 2A0

