The ForTI card

The ForTI card is a sound card that features four TMS9919 sound chips, identical to the one located within the console. The card-controlling software is written in Forth, wich may be the reason for the card's name.

Hardware
Bus interface
Power supply
Address decoding
TMS9919 wiring

Software
Low-level access
High level program

Hardware

Bus interface

The card interfaces with the PE-box bus in the prescribed manner: a 74LS245 buffers the data bus, and three 74LS244 take care of the address and control bus. A 74LS125 is used to buffer the card-generated RDBENA* signal that will enable the data bus drivers in the PE-box connection card and firehose connector. The remaining 3 gates on the 74LS125 are not used.

            74LS244            
           +------+      
     A0>---|      |---A0    
      .    |      |   .  
      .    |      |   .    
      .    |      |   .    
      .    |      |   .   
      .    |      |   .     
     A7>---|      |---A7        
      Gnd--|EN*   |         
           +------+    
            74LS244     
           +------+   
     A8>---|      |---A8     
      .    |      |   .       
      .    |      |   .         
      .    |      |   .       
      .    |      |   .                  
      .    |      |   .    
     A15>--|      |---A15       
      Gnd--|EN*   |      
           +------+      
            74LS244         
           +------+         
    AMA>---|      |---AMA
    AMB>---|      |---AMB
    AMC>---|      |---AMC
 MEMEN*>---|      |---Memen*
    WE*>---|      |---WE*        
CRUCLK*>---|      | (nc)         
CLKOUT*>---|      | (nc)         
   DBIN>---|      |---,            
      Gnd--|EN*   |   |        
           +------+   |      
            74LS245   |        
           +------+   |        
           |   DIR|<--'       
     D0<---|      |---D0            
      .    |      |   .        
      .    |      |   .    
      .    |      |   .    
      .    |      |   .                     
      .    |      |   .                     
     D7<---|      |---D7                
           |   OE*|<------+---CardSel*    
           +------+       |                            
                          |         
RDBENA*<------------------<|--Gnd
                      74LS125  
 

There is no CRU interface, and no memory to hold DSRs. Also, there is no LED to shine when the card is accessed.

Power supply

                  78L05                             All chips
               +----------+      +5V                 +-----+
+8V---+--------|Vin   Vout|----+-+----- ... --+------|Vcc  |    
      |        |   Gnd    |    | |            |      |     |
      = 0.1 uF +----------+ 47 = = 0.1 uF     =0.1uF |     |
      |             |       uF | |            |      |     |
     Gnd           Gnd         Gnd            '------|Vss  |
                                                     +-----+

Power supplied by a standard 7805 +5 volts voltage regulator, hoooked to the unregulated +8 volts line.

Each chip (including the sound chips) has a 0.1 uF bypass capacitor across its power lines, to damp oscillations.

Address decoding

The address decoding is performed with few TTL chips: a triple 3-bit NOR gate (74LS27), a dual 4-input NAND gate (74LS20) and two inverters from a 74LS04 hex-inverter chip. Together, they sample A0 through A5 to make sure the address is in the range >84xx, as well as Memen*, WE* (the card can only be written to), A15 (only even bytes are accessible), and the reserved address lines AMA through AMC.

 A1___ 
 A2---)>o--------,
 A3-'    74LS27  |
     A4___       |
    A15---)>o--, |
 Memen*-'      | |  _
      _        | '-| )
A0---| )       '---|  )o---CardSel* 
AMA--|  )o---|>o---|  )
AMB--|  )   '04  ,-|_) 74LS20
AMC--|_)         |
                 |
WE*-----,__      |
A5---|>o---)>o---'
  74LS04 |   74LS27
        Gnd

The resulting CardSel* signal enables the 74LS245 data bus transceiver, generates the RDBENA* signal, and enables one or the other sound chip.

The sound chips are not selected by a multiplexer though, but rather by the address lines A11 through A14 (active low). This means that it is possible to access several chips at a time, for instance to play the same sound on more than one chip (stereo, etc). As can be seen below, each address line is combined with the CardSel* signal via an OR gate (74LS32) to activate the CS* pin of a TMS9919 chip.

TMS9919 wiring

The wiring is common to the four chips for all pins but the CS* pin and of course the AudioOut pin. Pins D0 through D7 are connected to the data bus, pin WE* is held low permanently (why not use the WE* signal?), and the READY pins are connected direclty to the SysRdy* line in the PE-box bus.

The clock signal is generated by an onboard oscillating circuit, since the required 3.58 MHz signal is not part of the PE-box bus. The oscillator consists of a 3.58 MHz crystal, and three inverters, from the 74LS04 TTL chip (which leaves only one gate unused in this chip). The signal is then divided by 8 thanks to a 74LS161 counter. I'm not sure why this is necessary. Maybe so that sounds with a frequency lower than 110 Hz (the minimum with 3.58 MHz) can be programmed?

                             74LS161
       3.58 MHz             +-------+
 ,------|[]|------,  +5V--+-|EnP    |   
 | 1K2        1K2 |       +-|EnT    | 
 +-WWW-,    ,-WWW-+       +-|Rst*   |
 |     |    |     |       '-|LD*    |
 +-|>o-+-||-+-|>o-+---|>o---|Ck     |
 |      10nF    74LS04      |    RCO|
 = 33pF                   ,-|A    QA|
 |                        +-|B    QB|
Gnd                       +-|C    QC|--,
                          +-|D    QD|  |
                            +-------+  |
          ,----------------------------'
          |  +--------+
          '--|Ck Ready|---------->SysRdy*    
     D0------|D0      |+
     .       | .      ||
     .       | .      ||+
     D7------|D7      ||| 
        Gnd--|WE*     |||+
 A14---,     |        ||||    100 uF
,------=)>---|CS*  Out|----+---||------------<  #1
| A13---,    +--------+||| |            Gnd-'
+-------=)>---|CS*  Out|---|-+---||----------<  #2
|  A12---,    +--------+|| | |          Gnd-'
+--------=)>---|CS*  Out|--|-|-+---||--------<  #3
|   A11---,    +--------+| | | |        Gnd-'
'---------=)>---|CS*  Out|-|-|-|-+---||------<  #4
        74LS32  +--------+ | | | |      Gnd-'
                           = = = = 0.1 uF
                           | | | |
                           R R R R 10 Ohms
                           | | | |
                           '-+-+-+-Gnd

Finally, each AudioOut pin is filtered and applied to one of the four audio jacks, at the back of the card, thereby providing a quadriphonic output. A mixer adapter can be build with six 390 Ohms resistors to convert these four outputs into a stereo signal, with chips #1 and #3 making up one channel, and chips #2 and #4 making up the other.

        390 Ohms
#4|--+--WWW--------<
     '--WWW--,
#2|-----WWW--+-------------< Stereo 1
  
#3|-----WWW--+-------------< Stereo 2
     ,--WWW--'
#1|--+--WWW-------<

Software

As I mentionned, the card does not have any DSR memory. All the software must thus be loaded upon use.

Low-level access

Refer to this page for details on programming a TMS9919 chip. The target address changes according to the chip, remember that it is possible to access more than one chip at a time.

The memory map for all four chips is the following:

Address TMS9919#1 #2 #3 #4    Chip accessed
 >8400          +  +  +  +    All 
 >8402             +  +  +
 >8404          +     +  +
 >8406                +  +
 >8408          +  +     +   
 >840A             +     +
 >840C          +        +
 >840E                   +    #4 alone
 >8410          +  +  +   
 >8412             +  +   
 >8414          +     +   
 >8416                +       #3 alone
 >8418          +  +      
 >841A             +          #2 alone
 >841C          +             #1 alone
 >841E                        none (console chip alone)

NB The console TMS9919 answers to all addresses.
   The above pattern is repeated for addresses >8420-843E, >8440-845E, upto >87FE.

In most cases, you will only need to access one chip at a time, except to silence them all. The usefull addresses thus boil down to:

>841C: TMS9919 #1
>841A: TMS9919 #2
>8416: TMS9919 #3
>840E: TMS9919 #4
>8400: All
>841E: Console chip only

High-level program

A sophisticated Forth program is provided with the card, that lets you program music within a Forth "Screen". It lets you define:

If you're used to compose on music paper, it will seem a little complicated at first, but for programmers with little musical backgroud, the system is quite appealing.