Casiotone 401

the first Casio keyboard with automatic accompaniment & rhythm

This instrument from 1981 was Casio's first keyboard with accompaniment and (as far I remember well) also the world first keyboard with automatic single finger accompaniment.

The Casiotone 401 has 14 unusual, partly incredible dry semi- analogue preset sounds and 16 analogue rhythms; also the bass sounds nicely warm and seems to be based on a triangular wave. Unusual is the "hold" switch, much like the manual fingered chord mode (with chord memory) on newer keyboards, here also the melody voice notes hold until any other chord or key combination is played. The noble designed case is unusually heavy, because it is made of plastic coated steel instead of genuine wood. The costly internal speaker belongs to the best sounding ones I ever heard in a home keyboard.

main features:

49 fullsize keys
built-in big speaker (high quality with reasonable bass)
main voice polyphony 8 notes
chord polyphony 12 notes
volume knob
separate accompaniment/ melody volume balance knob
tempo knob
16 semi- OBS preset rhythms {rock, slow rock1, swing, bossanova, march1, waltz, rhumba, habanera | rock'n'roll, slow waltz, shuffle, samba, march2, rock waltz, beguine, mambo} (selected through 8 locking button switches + "variation" bank switch)
14 OBS preset sounds {organ, flute, oboe, clarinet, trumpet, violin, cello, piano, harpsichord, celesta, accordion, electric piano, funny, frog} (buttons with each a red status LED)
"casio auto chord" switch {off, fingered, on} (manual chord mode with rhythm off)
"chord" switch {rhythmic, continuous}
chord memory switch (chord stays held after releasing key when on)
"octave down" switch (transposes main voice 1 octave down, works only in chord mode).
rhythm "fill-in" and synchro buttons
switches for vibrato, vibrato delay, sustain, hold
main voice timbres based on 2 mixed stair shaped waveforms with different digital envelopes, those are differently low pass filtered through capacitors. Vibrato modulates CPU clock oscillator.
analogue percussion {base, snare, low tom, high tom, open cymbal, close cymbal, woodblock} which uses transistor noise for cymbal and snare
complex multi- chip hardware:

main voice CPU= "NEC D773G, K05266, Japan" (64 pin zigzag DIL)
accompaniment CPU= "NEC D8049C 084, M0Z076-049, Japan" (40 pin DIL, Intel MCS-48 with 2KB ROM)
I/O IC= "NEC D8243C, E01039-005, Japan" (24 pin DIL, Intel 8243)
chord section tone generator= "Texas Instruments TMS3615NS, MBS [triangle] 8048" (28 pin SDIL, socketed)
main voice DAC= "AMD AM6012PC, 8050FM" (20 pin DIL, 12bit)

tuning knob
mains operated
jacks for headphone, line out, aux in, volume pedal, sustain pedal, foot switch

eastereggs:

sound select button for 23 preset sounds with optional tone memory switch addable.

likely different vibratos and spread tone scale switch addable.

1 octave lower chord channel output.

notes:

Although on the case back this instrument is called CT-401, but its official name on the front panel was Casiotone 401. All preset sounds are selected through non- locking OBS push buttons with individual small red indicator LEDs; apparently Casio wanted to visually demonstrate here their use of advanced digital technology, since the similar looking OBS rhythm buttons employ still old fashioned locking switches. But all buttons and switches have still the classic 1970th home organ appeal with even different button colours per preset sound group; they all respond easily and feel quite noble. (The controls e.g. strongly resemble those on older Yamaha Electone organs of the A-, B-, C- and D- series.) The speaker grill has still the same style like with the Casiotone 201 (the world first Casio keyboard), although here it is dark brown instead of black. Impressive are also the massive count of 6 jacks on the back. Unfortunately there are no individual volume controls for rhythm and accompaniment (although these can be certainly added). My specimen came with the volume pedal Casio VP-1 and a stylish chromed original Casio keyboard stand.

Like Casio CT-410V, the main voice sounds of the Casiotone 401 consist of 2 mixed stair waveform suboscillators with different pulse patterns and different digital volume envelopes, those are (depending on the preset) muffled by different filter capacitors. Thus both instruments have the same kind of sounds, although unlike the rather dull CT-410V, many sounds of the 401 have a reasonably bright and partly incredible dry timbre, and when a key is trilled with sustain, each new note here occupies a new sound channel, which produces a great phasing sound and volume increase effect although this eats up polyphony. In the bass range many sounds turn into a more or less buzzy, sonorous purring drone, which is a characteristic style element of squarewave based instruments. These basses can resemble some of the famous POKEY sound effects on Atari XL homecomputers and are very different from the gradually duller and duller growing sine wave bass behaviour of average Yamaha FM keyboard sounds. (For further technical details about this hardware family also see here.)

By the use of simple low pass filters, many sounds play high notes too quiet. The "organ" is a slightly dull metal pipe organ timbre, which seems to be a muffled variant of the that classic multipulse sound. The bass range reminds to a wooden accordion. The "flute" is dull and resembles a sine wave tone. As expected, "oboe" is harsher; the bass range resembles a saxophone. The "clarinet" turns thinner during its attack phase. The "trumpet" sounds quite thin and barely recognizable (like with many squarewave based instruments). The "violin" sounds ok (especially with vibrato), while the "cello" is dull and could be also a dull tuba or the like. The "piano" has a too dull bass range; the "electric piano" is the same in slightly brighter. The "harpsichord" sounds ok. The "celesta" is a musicbox sound that sounds unrealistic by a too slow attack phase; the timbre resembles a flute. The "accordion" has a fluttering attack phase (quickly quieter and louder again, more like a violin) and sounds too dull and plastic- like. The "funny" preset is a sort of rough digital slap bass (creaky picked e-bass) which sounds like "ennng!" (possibly it was initially intended to be called "funky", but sounded to artificial for this?). An even harsher variant of this is called "frog" (although it doesn't sound at all like one), which makes a wonderful rough and incredible dry bass noise. When sustain is switched off, all sounds stop almost immediately after releasing the key, and the sound presets itself also contain neither vibrato nor tremolo. The sustain duration depends on the preset sound. The vibrato is quite fast (6Hz) and its intensity depends on the preset sound. When "vibrato delay" is additionally pressed, the vibrato waits 1s before it starts (vibratos affect also sustain); with vibrato off this button does nothing. Very unusual is the "hold" switch, which makes the melody section of the keyboard behave like the manual fingered chord mode on most newer keyboards; the notes of any pressed key or key combination (e.g. chord) are held after key release, and after all keys are released, any newly pressed keys stops the old held notes and instead plays and holds the new ones. Apparently the algorithm of the manual fingered chord with chord memory was re-used for this. Despite there are nicely big semi- OBS preset sound buttons, they are no good realtime sound control since they mute held notes while pressed and cause a delay of about 0.3s after button release before held or new notes will sound again.

The percussion are typical soft analogue home organ drums; cymbals and snare are likely made from transistor noise. The patterns also include beside the usual stuff also a few nice latin rhythms and the "fill-in" button inserts a fill-in pattern (including accompaniment), which depends on the selected rhythm. Unusual is that during rhythm not only the "start/ stop" LED lights up during the first step of a bar, but both the red "start/ stop" and the (at that time expensive?) green "synchro" LED dimly flash during every step. Despite the locking button switches, the semi- OBS preset rhythm buttons switch a playing rhythm only at the end of the bar, which limits their use for realtime play tricks.

Unlike later Casio instruments, the accompaniment here was not yet called "Casio Chord", but "Casio Auto Chord" (abbreviated "CAC"), and it also did not feature yet an arpeggio. But the accompaniment of this historical instrument already accepts any disharmonic note combinations and not just those few ones that establishment has defined to be "chords". Apparently there was already more software behind than with the archaic Antonelli Star 2379, because the bass pattern here only plays its note sequence when it recognizes chord- like key combinations, while it plays only one note during wild clusters or a single key press. Unusual with the 401is the "chord" switch; when set to "rhythmic", you get the usual automatic bass and chord accompaniment, but when set to "continuous" only the bass plays automatic the note of the lowest pressed chord section key, while all keys of the chord section otherwise are in manual organ chord note like with rhythm off. The accompaniment patterns are made from a monophonic bass line (some with walking bass) and simple staccatos of the current chord (or note combination). The chord voice is a plain squarewave organ tone, while the bass is a dull and warm organ bass tone with sustain that reminds to a dull Roland TB-303 bass and may be made from a triangular wave. With rhythm off there is a manual chord mode (12 note polyphonic!) with bass on the lowest played key. Unfortunately also here the notes always stay within 1 octave, thus it is no real key split. Regard that with this ancient Casio the chord section will not play any sound when the "chord" switch is set to "rhythm" while rhythm is off. With the "memory" switch the current chord is held after key release (with or without rhythm). Moving the "chord" lever switch slowly during rhythm varies the chord volume, which can be also used musically.

circuit bending details

The Casiotone 401 uses the main voice CPU NEC D773G, which is an early member of the D77xG family. The accompaniment CPU "NEC D8049C 084" is an MCS-48 microcontroller with 2KB ROM (I dumped it) that controls through I/O IC NEC D8243C the chord section tone generator "Texas Instruments TMS3615NS", which is socketed by unknown reason. Because Casiotone 301 (same hardware without accompaniment) came first, it may be that a shortage of that IC prevented finishing the 401, so the TMS3615NS could be installed last when the rest was already completed.

The 401 is quite heavy (about 10kg?) because despite its case looks almost perfectly like wood, it is mainly made of plastic coated sheet steel. (Casiotone 201 and 202 were of genuine wood.) In spite of this it is not as robust as it appears, because at the corners of the side pieces (made of pressboard?) the plastic woodgrain tends to peel off. I had to hotglue mine back into place, but this sheet plastic stuff also cracks off easily when accidentally folded to hard. I bought my specimen in very dirty state; the whole case was severely sticky especially at the bottom. I am not sure whether it was fatty kitchen dirt or if anybody had tried to apply furniture wax on the case despite it is not of wood. I removed most of that gunk with a paper towel and vegetable oil (stay away from rubber parts) followed by water with dish washing soap, but it still feels sticky on hot days. The high quality speaker sits in an own thick plastic compartment of that even the cable hole was sealed with glue to turn it into a perfectly closed box. It is driven by a large hybrid amplifier module that is screwed to a sheet metal heatsink welded to the case bottom. This makes it very unpleasant to remove the mainboard because you have to unscrew the module and mess around with (possibly poisonous) old heat conductive paste.

The dirty speaker grill had to be removed also for cleaning, because someone spilled cacao or similar on it.

Be careful with the 5 metal lashes with screw holes those hold the rear part of the control panel from inside.

With case opened, these lashes are sharp as razor blades and easily damage the panel and PCB traces; with mine a preset sound LED failed by an accidentally cut trace after working on the opened keyboard. After soldering the trace I glued ribbon adhesive tape over the sheet metal lashes to make them less dangerous.

After opening, a lot of broken small black rubber rings fell out of the case - likely they belonged under the piano keys somewhere for damping (keys feel a little loose), but turned brittle by oily room air or ozone. The quite complex analogue hardware of this instrument has the size of an old style PC mainboard and contains many discrete components for the analogue percussion. The percussion have individual trimmers for their decay time. The main IC numbers and sounds have some similarities with the smaller Casio MT-40. The power amplifier is a quite big hybrid module. Unusual is that the fingered chord is up to 12 note polyphonic with additional monophonic bass, while the main voice polyphony is only 8 notes.

I initially had no service manual of the 401, but got a photocopy of Casiotone 403, which was of great help to understand the general hardware architecture, because it has the same accompaniment hardware combined with the main voice section of Casio MT-60 (first 403 models had CPU version D776G with external bugfix circuitry). They are a good didactic example how Casio in early instruments made several different CPUs cooperate and combine their keyboard matrices rather than the centralized master-slave approach found in later hardware, and how despite almost self-contained CPUs often minor functions like clock rate conversion or preventing wrong key press order during preset sound selection used a crazy amount of "glue logics" ICs cluttering up the mainboard. Later I found the service manual PDF of Casiotone 401.

The preset sounds are routed through switchable lowpass filters. The cutoff frequencies are set by main voice CPU pins 42..47 (IO-31, IO-4, IO-5, IO-6, IO-7, IO-8). In plain mode all filters are disabled.

cutoff frquency SW1 SW2 SW3 SW4 SW5 SW6 SW7 preset sound

plain X X X accordion, frog, funny, harpsichord

0.83 kHz X X X X piano

1.12 kHz X X X organ, flute, cello

1.8 kHz X X X oboe, clarinet, elec. piano

3.1 kHz X X trumpet, violin, celesta

SW1..SW7 are analogue switches in the filter schematics. The actual control line bits on main voice CPU out pins apparently differ; they seem to be mainly designed to identify each preset sound number. There are additional intermediate bits F1..F5 in a table below the handdrawn looking schematics (which unfortunately is cropped and fuzzy in the service manual PDF), those don't seem identical with SW1..SW7.

The vibrato LFO modulates the 566 kHz clock speed of the main voice CPU D773G with a 6Hz triangular wave. This is bizarre because the main voice CPU already contains digital vibrato, which is not used. Delayed vibrato is implemented by an analogue circuit that gets resetted when there is no sound coming out of the DAC. Perhaps this was the only reason form this complicated solution, or digital vibrato was still missing in a prototype of the D77xG. The earlier release of Casiotone 301 without the (in 401 still socketed) chord IC may hint that Casio finished the 401 last-minute when functional chip features were still uncertain.

Someone e-mailed me that in his Casiotone 701 the rare DAC IC "AM6012PC" died and that he successfully replaced 2 of them with the modern type "Analog Devices DAC312". It is unknown if these generally suffer of ageing, but another e-mail confirmed that they tend to die. The 401 has the same part connected with its main voice CPU, while the direct successor Casiotone 403 already had the DAC hybrid "EXK-SIOL0025C" (12 pin SIL) - possibly to improve reliability.

The microcontroller based bass accompaniment with tone generator is described in US patent 4561338. There it uses for each 2-bar bass pattern 16 ROM addresses (4 bit) with 5 bit bass note data (31=silence) and 5 bit percussion data (1 bit per sound). When during 2nd bar any chord key is pressed or held, the 4th address bit of the bass note counter resets and so repeats the 1st bar. The accompaniment CPU outputs a monophonic squarewave tone and trigger pulse for the analogue bass envelope circuit, and trigger outs for analogue percussion. In the actual instrument it also controls the chord tone generator TMS3615NS through an I/O IC.

Danger!: The IC TMS3615NS employs a +15V high supply voltage, which may destroy other ICs when accidentally shorted. Thus stay away from pin 4 and do not blindly poke around in the circuitry; also some logic ICs use this voltage.

keyboard matrix

Despite the keyboard matrix layout of the main voice CPU D773G is generic D77xG (i.e. preset sounds selected through keyboard keys), the sound select button was omitted and instead simulated by 14 LED-lit OBS preset sound buttons connected through logic gates, those feed the corresponding KI# lines during KO# with correct timing. This costly way was likely chosen to give it a more traditional home organ appeal to customers those felt scared off by the Casiotone 201 user interface upon a time that was not ready for sound bank instruments.

SK2 (19 KI2)	SK3 (20 KI3)	SK4 (21 KI4)	SK5 (22 KI5)	SK6 (23 KI6)	SK7 (24 KI7)		CPU pin
in 1	in 2	in 3	in 4	in 5	in 6	in / out
					O. piano	out 1	33 KC1
		O. elec. piano	O. frog	O. funny		out 2	27 KC6
O. accordion	O. harpsichord	O. violin	O. trumpet	O. celesta		out 3	26 KC7
	O. organ	O. flute	O. cello	O. oboe	O. clarinet	out 4	25 KC8

Unfortunately this wastes 9 of the 23 preset sounds buried in its CPU. I haven't analyzed the matrix by myself, but found info in Robin Whittle's great essay "Modifying the Casiotone Instruments" and finally the Casiotone 401 service manual. See Casio M-10 key matrix for the preset sound names of the remaining key.

Apparently a hi pulse on pin 34 I-2 informs the main voice CPU when a preset sound button is pressed. In fingered or single finger chord mode the CPU inputs KC1..KC3 get disconnected by gate logics to prevent sounding the main voice by chord section keys.

Matrix eastereggs are selected by each a locking switch in series to a diode at the following CPU pins:

memory set

Casio MT-30

memory set (silent)

This does the same like the normal 'memory set' but omits sounding a demo note.

tone memory 1

tone memory 2

tone memory 3

tone memory 4

vibrato

The vibrato circuit of the 401 modulates the clock speed by analogue means and so apparently does not use the digital vibrato of the main voice CPU. Thus combining both may possibly produce a richer sound.

mild vibrato

This vibrato has less depth.

slow vibrato

When vibrato is on, this switch slows it down.

octave down

This switch normally works only in chord mode. Likely it can be rewired or bypassed to work always.

spread scale

dissonant howl

These strange C2 key doublets use a different sound channel than their normal versions and change pitch and timbre when held or trilled during play of other notes; it also depends on vibrato and other settings. Sometimes each strange key press plays the note in different octaves. Attention: I haven't tried out these eastereggs, thus I don't know if they are correct.

control panel matrix

This incomplete control panel matrix is based on the Casiotone 401 (initially also 403) service manual. While the output (KC) pins come from the main voice CPU, the input pins (SW) here are sensed only by the accompaniment CPU D8049C (which syncs itself to them through pin 39 waiting on the KC-1 pulse). Robin Whittle claimed in his essay, that the preset sound buttons of the 401 simulate to the main voice CPU the classic 'set' switch + keyboard key selection method (like with Casiotone 201) using logic ICs. I expect that also here the D8049C is involved; when removed, the buttons don't respond and the instrument stays mute.

Panel LEDs are controlled through logic gates from main voice CPU pins 42..47, those also switch the fixed filters.

35 SW1 36 SW2 37 SW3 38 SW4 <- accomp. CPU pin

in 1 in 2 in 3 in 4 in / out CPU pin

R.
start/stop R.
synchro R.
fill-in R.
variation out 1 33 KC-1

C.
continue C.
memory on C.
casio chord C.
fingered chord out 2 31 KC-2

R.
bossanova R.
habanera R.
rock R.
rhumba out 3 30 KC-3

R.
slow rock 1 R.
swing R.
march 1 R.
waltz out 4 29 KC-4

out 5 28 KC-5

out 6 27 KC-6

out 7 26 KC-7

out 8 25 KC-8

legend:

underlined = function needs locking switch (i.e. stays active only so long the switch is closed)

R. = preset rhythm

C. = chord

orange
background = easteregg (unconnected feature)

pinout D8049C-084

The "NEC D8049C 084" (40 pin DIL) was the accompaniment CPU of the first polyphonic Casio keyboards with rhythm. It supports a keyboard matrix for chord section and control panel, and has separate outputs for 7 analogue percussion triggers, bass envelope trigger (for external analogue decay circuit) and 5 data pins those can be demultiplexed in an I/O port IC "NEC D8243C" to output a monophonic squarewave bass tone and control the chord tone generator "Texas Instruments TMS3615NS". Unlike later accompaniment CPUs it does not control a passive main voice sound IC, but reads 4 of the main CPU keyboard matrix outputs to sense chord section keys. To prevent these keys from sounding also the main voice, there need to be external AND gates in the lines to the main CPUs keyboard matrix inputs to disable them during chord play. The principle how this accompaniment CPU taps into a given keyboard matrix by listening to its output lines and intercepting inputs
is exactly the same like what midi retrofit kits for non-midi keyboards do.

Technically the "D8049C xxx" is a generic microcontroller of the well documented Intel MCS-48 family, so everything it does is entirely controlled by software of its internal 2KB ROM (I dumped it). So it makes no sense to write down a combined pinout for all software variants of D8049C because they differ too much. This pinout describes software number "084", based on the Casiotone 403 service manual and Intel MCS-48 datasheets.

pin name purpose

1 TO clock out 1MHz

2 XTAL clock in 3MHz

3 XTAL2 (not used)

4 /RESET reset

5 +VDD supply voltage +5V

6 /INT tempo clock in

7 EA (wired to ground)

8 /RD (not used)

9 /PSEN (not used)

10 /WR (not used)

11 ALE (not used)

12 DB0 key matrix in

13 DB1 key matrix in

14 DB2 key matrix in

15 DB3 key matrix in

16 DB4 key matrix in

17 DB5 key matrix in

18 DB6 key matrix in

19 DB7 key matrix in (chord switch /on)

20 GND ground 0V

21 P20 chord data out (D8243C pin 11)

22 P21 chord data out (D8243C pin 10)

23 P22 chord data out (D8243C pin 9)

24 P23 chord data out (D8243C pin 8)

25 PROG chord data out (D8243C pin 7)

26 +VDD supply voltage +5V

27 P10 bass envelope trigger out

28 P11 hihat trigger out

29 P12 cymbal trigger out

30 P13 low conga trigger out

31 P14 high conga trigger out

32 P15 claves trigger out

33 P16 snare trigger out

34 P17 base drum trigger out

35 P24 key matrix in SW1

36 P25 key matrix in SW2

37 P26 key matrix in SW3

38 P27 key matrix in SW4

39 T1 key matrix sync in (main CPU pin 33 KC1)

40 VDD supply voltage +5V

The tempo clock input to pin 6 is externally produced by a PUT (Programmable Unijunction Transistor) oscillator, which frequency depends on the tempo potentiometer. Rhythm can start only at the falling edge of it, therefore oscillation is forced into the next lo phase by a pulse from D8243C pin 15 to start rhythm immediately by a start button or chord key press (synchro start). The clock frequency for TMS3615 is divided from pin 1 output through a complicated gate and flipflop circuit (TC4001, 2x TC4013, TC4510BP).

The I/O expander NEC D8243C converts the chord data into this:

pin name purpose

1 P50 chord E bit out

2 P40 chord C bit out

3 P41 chord C# bit out

4 P42 chord D bit out

5 P43 chord D# bit out

6 GND ground 0V

7 PROG data in (D8049C pin 25)

8 P23 data in (D8049C pin 24)

9 P22 data in (D8049C pin 23)

10 P21 data in (D8049C pin 22)

11 P20 data in (D8049C pin 21)

12 GND ground 0V

13 P70 green led drive out

14 P71 red led drive out

15 P72 accompaniment /start out

16 P73 bass tone out

17 P63 chord B bit out

18 P62 chord A# bit out

19 P61 chord A bit out

20 P60 chord G# bit out

21 P53 chord G bit out

22 P52 chord F# bit out

23 P51 chord F bit out

24 VDD supply voltage +5V

Pin 16 outputs the bass tone (monophonic squarewave) that is externally mixed with an analogue envelope controlled by D8049C pin 27; the tone is held so long it stays hi and then decays with capacitor envelope. The TMS3615NS produces 12 tones (full polyphonic notes of 1 octave with sustain) corresponding to the levels on the 12 chord bit outputs.

pinout TMS3615

The OMTS "Texas Instruments TMS3615NS" (28 pin SDIL, official name "Octave Multiple Tone Synthesizer") is used as a chord generator in the first polyphonic Casio keyboards with accompaniment. The IC derives from its clock rate the 12 tones of an octave (and a 13th tone) and outputs each of them so long the corresponding note input pin is hi(?). When the note stops, it falls silent with a decay envelope (sustain) that can be adjusted through an analogue control voltage input. This ancient PMOS IC was meant as an octave generator in full polyphonic 1970th home organs with very little digital control; a sign of its age is the unusually high +15V supply voltage (minimum +12V), which prevented easy battery operation and so made Casio soon abandon it. It can output 2 adjacent footages and sustain of the output is inplemented by external 1uF capacitor at each input, i.e. they are indeed VCA control voltages rather than digital. Pin 3 changes the pullup impedance of the key inputs and thus sustain length. The reset input is only used to synchronize the internal oscillators to control the phase in relation to other tone generators in the system. The clock out at half frequency was intended to daisychain these ICs in keyboards with multiple octaves. Interesting is that the TMS3615NS outputs 2 footages (e.g. for drawbar organs), thus an additional 1 octave lower version of the accompaniment (not used by Casio) exists on pin 16'OUT as an easteregg.

The inner working of this IC is thoroughly described in patent US4358982 (priority date 1979 | thanks Traktor for info). The signal processing behind the frequency dividers is fully analogue, including a large bank of VCA multiplying the key voltage (and volume envelope) with each drawbar partial tone. For these, the patent detailedly explains voltage stabilization and a special automatic DC offset voltage compensation circuit based on clocked (class D PWM?) chopper amplifiers to prevent key click and distortion.

This pinout is based on the Casiotone 403 service manual and TMS3615NS datasheet; in 403 schematics the IC is named "TMS 3615-25NS, RI-103" and also a 403 hardware photo shows "-25NS", so it may be a variant (clock speed like with RAM?). The variant RI107 outputs squarewave with 50% duty cycle, while RI103 has 25% at 8'OUT.

warning: A bizarre bug in the TMS3615NS datasheet of novermber 1981 garbled its pinout page, so all its pin numbers up to 15 are 1 too high and the rest 1 too low (verified by Casiotone 401 PCB photo). Apparently they wrote it down wrongly from a chip package drawing (absent in datasheet) because GND is indeed pin 13, and 14 and 15 unused, so I shifted them to to fill in the unused pins here. Likely TI initially planned to produce this chip in a shorter package, but 24 pin SDIP was not available.

On Vintage Chip I saw indeed a 24 pin DIL version for sale. A hand drawn pinout on Flickr by synx508 indicates that in Roland HP-60 the SDIL version of TMS3615NS has the same pinout like mine, suggesting that the other pinout was possibly used by an 28 pin DIL version.

pin datasheet pin name purpose

1 2 NC (not used)

2 3 16'OUT? audio current '16 out (not used)

3 4 SUS BIAS sustain lenght control voltage in

4 5 VSS +VCC supply voltage +15V

5 6 K8 note F# control voltage in

6 7 K9 note G control voltage in

7 8 K10 note G# control voltage in

8 9 K11 note A control voltage in

9 10 K12 note A# control voltage in

10 11 K13 note B control voltage in

11 12 RESET OUT (not used)

12 13 CLOCK IN clock input (118.5 kHz)

13 14 VDD GND ground 0V

14 1 NC? (not used)

15 28 NC? (not used)

16 15 CLOCK OUT? clock/2 out (not used)

17 16 RESET IN reset

18 17 K1 note B-1 control voltage in (not used)

19 18 K2 note C control voltage in

20 19 K3 note C# control voltage in

21 20 K4 note D control voltage in

22 21 K5 note D# control voltage in

23 22 K6 note E control voltage in

24 23 K7 note F control voltage in

25 24 STB IN internal stabilizer control in

26 25 STB OUT internal stabilizer control out

27 26 8'OUT audio current '8 out

28 27 NC (not used)

In Casiotone 403 schematics the pin 3 SUS BIAS is between 2 resistors forming a voltage divider 33k to +VCC and 22k to GND. Pin 26 STB OUT goes to the "+" input of an op-amp and 100k pulldown to -15V. Pin 25 STB IN goes to a 3.3uF electrolytic cap against +15V and through a 220 Ohm resistor to the output of that op-amp. (Its "-" input is on GND.) According to datasheet this does automatic gain control to compensate temperature drift of the analogue circuit. If absent, STB IN should be wired to +VCC and STB OUT open. The datasheet suggests that against ESD damage(?) the pins SUS BIAS and STB IN should be connected through each a 0.5 megohm resistor to +VCC on the same PCB.

A Casiotone 401 variant without accompaniment section was released earlier (1980) as Casiotone 301 (seen on eBay - likely rarer than the 401). This CT-301 had an identical case beside that it lacks the 4 white switches, the letter row above leftmost keys, the fill-in button and has only 4 jacks. Also the control panel sections are in different order (power switch, 16 preset sound buttons, vibrato switches, 8 rhythm buttons, rhythm start/ stop & synchro switches, 3 knobs) and employ the 2 black toggle switches here for rhythm start/ stop and synchro instead of the missing sustain and hold functions. Also the orange rhythm select switch apparently does something else, thus there are only 8 OBS rhythms. The same main voice CPU was also used in the simple Casio M-10. The accompaniment hardware of the 401 was later used in Casiotone 403 (seen in service manual) with main voice like Casio MT-60.

*removal of these screws voids warranty...*

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