::: Area #102 (BBC Echo)
Message: #1293 (Read 1 time, has 0 replies, 3492 bytes)
Date   : Wed May 20 11:50:55 1998
From   : Jonathan Graham Harston of fidonet#2:254/27
To     : All
Subject: Subject: Running a BBC Without A Keyboard

JGH> I'm going to do some fuller testing of this.  I have occasionally
JGH> seen a keyboardless Beeb just hang with the cursor in the top
JGH> lefthand corner of the screen doing nothing.  However, it's managed
JGH> to get into MODE 0, so it must have read the kaybaord links by the
JGH> time it's got there.

Following up my own posting, I've done a bit of testing.

With the 8 BBCs I have here, I unplugged the keyboards and switched them
on.  They did the following:

6 of them: went into MODE 0, blank screen, cursor in the top corner
2 of them: went into MODE 0, displayed BBC Computer... etc. and tried to
	    do a Shift-Break

(I haven't tested the Master or Compacts yet)

Of the six that just sat there, if I carefully plugged the keyboard back
in, they then displayed BBC Computer... etc and tried to do a Shift-Break
(from Network if it was fitted).

All eight machines are issue 7.

Investigating the Reset routine, the keyboard links are read here:

; Read keyboard links
DA10   INX         ;X=9
DA11   TXA         ;A=X
DA12   JSR &F02A   ;interrogate keyboard
DA15   CPX #&80    ;for keyboard links 9-2 and CTRL key (1)
DA17   ROR &FC     ;rotate MSB into bit 7 of &FC
DA19   TAX         ;get back value of X for loop
DA1A   DEX         ;decrement it
DA1B   BNE &DA11   ;and if >0 do loop again
		    ; on exit if Carry set link 3 made
		    ;link 2 = bit 0 of &FC and so on
		    ;if CTRL pressed bit 7 of &FC=1
		    ;X=0

; Read from keybaord
F02A   LDY #&03    ;stop Auto scan
F02C   STY &FE40   ;by writing to system VIA
F02F   LDY #&7F    ;set bits 0 to 6 of port A to input on bit 7
			 ;output on bits 0 to 6
F031   STY &FE43   ;
F034   STX &FE4F   ;write X to Port A system VIA
F037   LDX &FE4F   ;read back &80 if key pressed (M set)
F03A   RTS         ;and return

I can't see anything here that would stop the machine with no keyboard,

Further along, though, is:

	; A=&7F
DA77   CLI         ;briefly allow interrupts to clear anything pending
DA78   SEI         ;disallow again N.B. All VIA IRQs are disabled
DA79   BIT &FC     ;if bit 6=1 then JSR &F055 (normally 0)
DA7B   BVC &DA80   ;else DA80
DA7D   JSR &F055   ;F055 JMP (&FDFE) probably causes a BRK unless
		    ;hardware there redirects it.

This vector is to allow plug-in hardware to redirect the Reset routine by
forcing an interupt on Reset.  If unplugging they keyboard leaves the
keystrobe interupt line enabled, then this will for a call to the vector.
With no hardware to direct it, it will probably hang.  Which is what is
happening.

So, holding the keystrobe line high should do it.  A 4.7k resistor between
pin 14 (CA2) and pin 15 (+5v) should do it.  As these are adjacent pins,
you can make a little carrier from a two-pin molex plug to just plug onto
the pins.

I'll investigate and follow up.

   J.G.Harston (JGH BBC PD Library) <mailto: jgh@arcade.demon.co.uk>
Expertise in: BBC+Master / Z80+6502 / CoPro+Tubes / Econet / SJ Servers
  Walkley / Planning / Environment / Highways / Transport / Devolution
70 Camm Street, Walkley, SHEFFIELD S6 3TR  -+-+-  Herder of cats
	   http://homepages.nildram.co.uk/~amilton/walkley.htm
	    (( Anti-UCE address added by Arcade, not by me ))
--- ARCbbs RISC OS [1.64á32c]
 * Origin: Arcade - The Definitive Acorn BBS +44 181 655 4412 (2:254/27)