IMSAI 8080‎ > ‎

IMSAI 8080 page 4 – serial I/O

I/O for the IMSAI

The 1976 Processor Technology Corporation 3P+S Input / Output Module Revision A.


I love the trace layout on this board. You can just imagine Ingram and Marsh (and Felsenstein) in their Berkeley garage laying out the pads and traces by hand. It has a distinctive, human touch.

Here's what I know about the board so far:
  • None of the tantalum caps are shorted
  • The 7805 produces 5V
  • The trace to pin 24 (ø2 clock) is cut and jumped to pin 49 (clock).
  • I couldn't make sense of the jumper settings on the card!
After setting up the card for port 00-03 and RS-232 output, I made it to here:


A short program sends the active user input port switches to port 01 (and the front panel display). I can see the bits on the scope. However, there seemed to be memory corruption issues with the 3P+S card installed, especially at $0000 and $0001. I removed the jumper to pin 49 and repaired the trace to pin 24. It looks as though things have improved. 

Alas, a short while later, the card stopped sending any output and the memory became flakey again on all pages at addresses 00 and 01. It seems that the card is creating a bus conflict. I suspect it is IC 22, a DM8836 (N8T380N) "Quad Bus Receiver" (quad NOR) that controls the 3P+S DI lines. When the CPU signals an I/O port read (SINP) simultaneous with a data bus read (P DBIN) through IC 22 along with the jumpered I/O addresses on the address lines, the N8T97 buffers driving the DI lines are pulled out of a high impedance state. Most of the output pins on IC 22 are floating around 1.8-2.3, and as a result, the card's buffers are kept active on the DI lines. Unfortunately, this chip is incompatible with 7400 equivalents, so it will require a special order.

Update: replacement Signetics N8T380N received, installed, and card function restored. The IMSAI was last seen sending ASCII to a Raspberry Pi's GPIO at /dev/ttyAMA0. 

3P+S settings


Currently, I have the 3P+S jumper settings configured as follows:

Area G (EIA inputs)

TBE (transmitter buffer empty) to D7

RDA (receiver data available) to D6


EIA 1 IN (received data) to UART. Connect jumper between 1 (just below "IC10") and Rin.

EIA 2 IN (carrier detect) to DI0 (data in bus) - jumper between 2 and C0


OE, FE and PE no jumpers installed


Area E 

9600 baud:

GGGV GVVV VVVV

        G = ground (row 1), V = +5V (row 2)


        Alternatively, the baud rate can be set to 300 baud:

GGVG VGVG GVVV


Area H - word length and stop bits (8N1)

SBS to ground - stop bit select. One stop bit if low.

Since WLS1 and WLS2 (word length select) are not connected they are held high. This gives 8 bits per character.

Since PI (parity inhibit) is not connected and thus held high, it gives NO PARITY.

Area J (EIA outputs)

EIA 1 OUT to UART. This is pin M on the J1 slot.


Other jumper settings:

Area A

All jumpered to ground, which means I/O ports 0x00-0x03


Area B (order of response of four I/O channels)

Left Jumpered to Right, which means channel order is c,d,a,b

A1   A0         channel

0     0         C 0000 0000 - status channel

0     1         D 0000 0001 - serial, handles data to / from UART

1     0         A 0000 0010  

1     1         B 0000 0011


Area C

Right jumpered to center, which means hardware UART setup (not programmable setup)


Area D 

Center to Right, which means that current loop is disabled


Area F (peripheral control driver)

nothing


The RS-232 output is on pin M of J1, the input is on pin K of J2, and the signal ground is pin 11 on J1.

Serial loader

Now that the serial card is up and running, I'm using the following loader to transfer program binaries from a Raspberry Pi to 0000H on the IMSAI. The 20-byte loader program is toggled in at $1C00, the last 1k in my 8K of RAM. The program outputs the current byte being written to the programmed output LEDs. 

     1                                  ;; ================================================
     2                                  ;; LOADER_1.asm
     3                                  ;;
     4                                  ;; Load code at 0000H from 3P+S serial card
     5                                  ;; card is configured for I/O on port 01
     6                                  ;; status byte on port 00. Status bit C7 is
     7                                  ;; TBE and status bit C6 is RDA.
     8                                  ;;
     9                                  ;; asm8080 cross-assembler
    10                                  ;; asm8080 XX.asm -P -l
    11                                  ;; ================================================
    12
    13
    14            00 00         STATUS  EQU 0
    15            00 01         PORT1   EQU 1
    16            00 80         TBE     EQU 080H        ; transmitter buffer empty bit is D7
    17            00 40         RDA     EQU 040H        ; receiver data avail bit is D6
    18            00 FF         PANEL   EQU 0FFH
    19
    20            1C 00                 ORG     1C00H           ; put in top 1K
    21                                  
    22 10    1C00 11 00 00              LXI     D,0000H
    23 10    1C03 DB 00         LISTEN: IN      STATUS          ; check for incoming data
    24  7    1C05 E6 40                 ANI     RDA             ;   using RDA bit
    25 10    1C07 CA 03 1C              JZ      LISTEN          ; no data, keep listening
    26 10    1C0A DB 01                 IN      PORT1           ; read serial input
    27  7    1C0C 12                    STAX    D               ; store in memory
    28  5    1C0D 13                    INX     D               ; next memory location
    29  4    1C0E 2F                    CMA                     ; output current byte
    30 10    1C0F D3 FF                 OUT     PANEL           ;     to front panel LEDs
    31 10    1C11 C3 03 1C              JMP     LISTEN
    32                                  END


*******************************************************************************
                                 Symbols table
*******************************************************************************

Names           Types   Values
-----           -----   ------
STATUS          EQU     00000h
PORT1           EQU     00001h
TBE             EQU     00080h
RDA             EQU     00040h
PANEL           EQU     000FFh
LISTEN          Label   01C03h


The Pi is set up with an RS-232 shield that runs off the GPIO /dev/ttyAMA0 pins. At some point, I must have disabled the console output on /dev/ttyAMA0 in /etc/inittab. Once /dev/ttyAMA0 is configured with stty, the loader is started and the binary transferred:

cat PROGRAM.bin > /dev/ttyAMA0

I transferred Steve Dompier's MUSIC OF A SORT program and data in about five seconds at 300 baud. The 212 byte 8080 binary is attached below. Whew, what a relief. Toggling in 212 bytes stinks! 

It seems that I'm on track in recreating the 1970's early microcomputing experience. Here's a reflection from the era taken from the February, 1976 issue of Dr. Dobbs: "The goal of about ninety percent of small systems owners appears to be to get their systems up and running with some kind of I/O and then procure enough memory to support a higher level language."


Page listing - "IMSAI 8080 pages"

DOMPIER.bin
(0k)
eric F,
Oct 1, 2016, 6:25 PM
v.1
DOMPIER.pdf
(125k)
eric F,
Oct 2, 2016, 10:48 AM
v.1