Spectrum Cassette, EAR and MIC port, Issue 2 and 3

Research about it I started when I tested some games on different Spectrum emulators: I found that games Abu Simbel Profanation and Rasputin wouldn't start play from the control selection menu.

This program is intended for authors of Spectrum emulators:

10 OUT 254,BIN 11101111
20 PRINT IN 254
30 OUT 254,BIN 11111111
40 PRINT IN 254
50 GOTO 10

For correct test do not press any key while running, and of course have no EAR input.

• If the output is 191,255,191,255 etc, you are on real Spectrum Issue 3.
• If output is always 191 or always 255, change the value in line 10 to BIN 11100111.
• If output is then 191,255,191,255 etc, then you are on Spectrum Issue 2.
• If output is still always 191 or always 255 you are on PC, on Spectrum emulator! (Warajevo gives something like 255,255,255,255,255,191,191,191,255,...)

Note: Issue 2 and Issue 3 are different versions of Spectrum's PCB, but real difference is actually version of ULA chip. Later PCB versions use same ULA as Issue 3, so behavior is also same.

The only correctly responding emulator which I found is zx32 by Vaggelis Kapartzianis: in this test this emulator acts exactly like an Issue 2, and also zx32 was the only emulator which correctly started Abu Simbel Profanation and Rasputin. (I tested x128 v0.8, Z80 v3.05, Warajevo v1.5, UKV, JPP, ZX-JAM, WSpecem, SP2 and VGASpec.)

By port 254,  the ULA chip uses the same pin for 3 functions: MIC output, EAR input and internal speaker drive. That's why OUT 254 or sound generation affects bit 6 when it is read by IN 254. 

In Z80.DOC by Gerton Lunter (which is the reference for most other documents) are some errors:

• "..bit 5 (value 64)..". This should be bit 6 - this has been noticed in the Spectrum FAQ, and is probably only a mistype, as the value is OK.
• The explanation of bits 3 and 4 by OUT 254 is not correct, because ULA does not have separate pins for EAR and MIC: the difference is that bit 4 produces louder sound (You may hear it from program above). And EAR is normally input, not output... but you may use it as output as well, as it is the same pin. Bit 3 is mostly used for setting correct voltage level on this pin by loading from tape. MIC and EAR sockets are separated only with resistors.

What is missing from Z80.DOC is the fact that not only the input level at EAR affects bit 6 by IN 254. Bits 3 and 4 from the previous OUT 254 also have an effect, and there is little difference between Issue 2 and 3, and it isn't something like the inversion of the EAR input.

    The difference between Issue 2 and 3 is:

• In Issue 3 is enough that bit 4 in OUT 254 is zero to get bit 6=0 in following IN 254.
• In Issue 2 both bit 3 and 4 must be zero for the same.

            Truth table:

Out b4	Out b3	In Is2	In Is3	Is2 V	Is3 V
1	1	1	1	3.79	3.7
1	0	1	1	3.66	3.56
0	1	1	0	0.73	0.66
0	0	0	0	0.39	0.34

Out b4 and Out b3 are output bits.  In Is2 is value of bit 6 by IN (254), after propriety OUT by Issue 2, and Is3 is same that - by Issue 3.     Is2 V  and  Is3 V  are voltage levels on pin 28 of ULA chip after propriety OUT,  without input signal on EAR. This is common pin for MIC, EAR and internal speaker.

I tested on which level on pin 28 input bit 6 changes from 0 to 1 or reverse. This is   exactly 0.7 Volts by Issue 2 and same by Issue 3 , and no inverting, no hysterezis:

Iinput bit 6 is 1 if voltage on pin 28 is over 0.7 Volt, else is 0 - by both Issues.

On hardware level  only difference between Issue2 and 3 is this: there are little higher voltages by Issue 2. You may see in table , by input combination: 0 1  that voltage values are near around that 0.7 V. 

By faster programs situation isn't so simple, there is some delay when output bit 4 changes from 1 to 0. And games are written mostly in assembler. To illustrate it here are 2 short assembler routines:

      ORG 45000
LD A,#18  ;OR #F8 e.g.
OUT (254),A
LD A,#08  ;OR #E8
OUT (254),A

TIMING  LD B,7 ;crucial value
DL     LD IX,0

 DJNZ DL
 IN A,(254) ;query state

 In this case IN A,(254) - output gives sometimes 255, sometimes 191. If you change 'crucial value' in line TIMING to a smaller one, then the result will be always 255, if delay is longer (B>7) then the result will be always 191. Of course this is valid only for Issue 3.  The  situation is something different by longer duration of high output level on port 254:

   ORG 50000
HALT  ;sync.
LD A,#18
OUT (254),A
HALT   ;delay 20ms
LD A,#08
OUT (254),A 

TIMING  LD B,107  ;crucial value
DL    LD IX,0    

DJNZ DL
IN A,(254)

   As you may see, after longer high level duration -  delay is also much longer.   So delay varies approximately from  180 T states (about 50 microsec.) to 2800 T states (about 800 microsec.) - depending from the duration of high level on port 254.   Explanation for this delay are capacitors connected between pin 28 of ULA and EAR and MIC connectors.
   By changing from 0 to 1 (bit 4 by OUT) no delay. 

In the ROM 'Beeper' subroutine, at address #03CF there is instruction OR 8, which is before OUT (#FE),A , and this routine is activated every time when you type character in Basic; And it is the reason (with all above) why IN 254 on an Issue 2 normally gives 255, and 191 on an Issue 3.

(Written somewhere in 1999...  Modified in August 2005, mostly grammar)

  Situation with emulators in August 2005:

Let authors correct me if I'm wrong somewhere. I tested only Windows emulators, newer ones. (except Z80 Win, which is from 1999, from respect to author of first serious Spectrum emul. for PC) and Fuse (Linux). Will not test more alphas like V 0.1 and similar...
Result is 10:7  for no support :-(

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