While thinking about ways to connect more hardware to my Altair 8800 Simulator I realized that the signals driving the DATA and ADDRESS LEDs, together with a few control signals (INP, OUT and WAIT), could be used to form a simple but effective I/O bus.
Such a bus could work very similar to the original S-100 bus and allow multiple external devices to be connected and accessed by existing emulated software with only minor changes to the emulator firmware.
When the emulator performs an OUT instruction it sets the A0-7 LEDs to the output address, sets the D0-7 LEDs to the output data and then switches the OUT LED on. External hardware connceted to the bus will see the low->high transition of the OUT signal, can see whether the proper address is present on A0-7 and if so read the data from the D0-7 signals.
When the emulator performs an IN instruction it sets the A0-7 LEDs to the input address, switches the D0-7 data lines to input mode and then switches the INP LED on. External hardware will see the INP signal and can can place the proper data on the D0-7 data lines for the emulator to read.
In both cases external hardware can pull the WAIT signal high to indicate that it needs more time to process the IN/OUT instruction. If so, the emulator will wait until WAIT goes back low before continuing program execution.
For a more detailed description of the timing and possible handshaking see the "Interfacing external hardware" section in the Altair 8800 simulator documentation.
While Input/output operations on this bus work very similar to the S-100 bus, there are several significant differences that make it impossible to connect original S-100 hardware to the bus. But schematics for a range of S-100 devices are available and some can re-created to work with this bus. The main limitations are:
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No common clock signal.
The Arduino Due emulates the instructions executed by the Intel 8080 (or Z80) processor but each emulated instruction takes a different amount of real time. Furthermore the time for each instruction does not match 100% the timing of the original processor, i.e. the emulator does not emulate the real 2MHz clock of the Altair. That means that the I/O bus can not provide a clock signal which is required by most S-100 cards for synchronous communication between the card and the processor.
However, using the INP/OUT signals combined with the WAIT signal it is possible to communicate asynchronously. For more specific information on how this communication works, see the "Interfacing external hardware" section in the Altair 8800 simulator documentation.
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Shared data lines for input/output
The S-100 bus had separate data lines for data coming out of the CPU (DO0-DO7) and data going into the CPU (DI0-DI7). This I/O bus has only one set of data lines (D0-7). The CPU switches the direction of these lines depending on whether an input or output operation is taking place. A device attached to the bus must make sure to only put data onto the data bus while the INP signal is high and the device's address is present on the address bus.
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No interrupt signals
The I/O bus at this point does not support interrupts. It may be possible to add support for interrupts but very little software actually uses interrupts and the effort necessary to support them outhweighs the benefits.
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No RAM/ROM access
The I/O bus works only for INP/OUT instructions. It is not possible to intercept memory read/write operations at this point.
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Signal levels
All signals on the bus are 3.3V level whereas the S-100 bus worked with 5V signal levels.
So far there are nine different types of cards that can be plugged into a backplane which connects to the Altair Simulator. Each card is described in more detail in a sub-folder with corresponding name:
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An LED output register card that provides 8 LEDs at address 255, equivalent to the 8-bit output register on the IMSAI-8080
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A serial port card which is equivalent to "half" a 88-2SIO card ("half" because it provides only a single serial port)
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A parallel port equivalent to a 88-PIO card
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A floppy disk controller equivalent to the 88-DCDD and 88-MD (minidisk) controllers that can use regular 5.25" and 3.5" floppy drives as well as Shugart SA-800 8" drives. An alternate firmware for this card also lets it function as an ICOM3712 or ICOM3812 disk controller.
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A Centronics printer interface equivalent to a 88-C700 card
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A cassette interface equivalent to a 88-ACR card
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A speech output card using an SP0256-AL2 speech processor
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A real-time clock card using a DS1302 timekeeping IC
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A SPI and I2C card to allow communication with more modern devices via SPI and I2C
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A Bus Monitor card to monitor all I/O operations (actual bus cards as well as devices emulated in the simulator)
Note that all cards include jumpers to configure their address. That allows multiple copies of the same card to be plugged in at the same time.
A short video that demonstrates the different cards can be seen here:
