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                                 CHAPTER 4

                              ______________
                              TOP SLOT BOARD



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4.1  GENERAL



The Organiser interfaces to the outside world through three slots:-

     1.  SLOT 1.  Side entry, top (nearest display)

     2.  SLOT 2.  Side entry, bottom

     3.  SLOT 3.  Top entry

Slots 1 and 2 are generally used for memory devices such as  datapacks  and
rampacks,  and  slot  3  is  normally  used  for  communications  or  other
interfaces.  Electrically  all  three  slots  are  very  similar,  and  the
distinction  is  for  ergonomic  reasons.   Internally  the three slots are
connected together as a bus, carrying 8 bit bi-directional data, power  and
control lines to select and control devices plugged in.

     This section describes the bus hardware in general terms,  and  should
be read together with the sections on specific devices and control software
to gain a full understanding of its operation.



     ____ _______ ___
4.2  SLOT CONTROL BUS


Figure 3.1 is a schematic of the slot control circuitry.  All  three  slots
have 16 connections.  In general they are connected as a bus, but there are
some minor differences in signals particularly to slot 3.  The slot signals
and their functions are listed below :-
    1. power rails
        0volts          (all slots)     system ground
        Vcc3            (all slots)     5 volt rail, switched under
                                        software control
        Vpp             (slots 1,2)     21 volt rail for programming
                                        datapacks.
        Vb              (slot 3)        system power rail for power
                                        in or out. 5.5 to 11 volts.

    2. data bus
        SD7-SD0         (all slots)     8 bit data bus from processor
                                        port 2

    3. control bus
        SCK             (all slots)     4 general control lines
        SMR             (all slots)     from processor port 6
        SOE_B           (all slots)
        SPGM_B          (slots 1,2)

    4. slot selection
        SS1_B           (slot 1)        3 control lines from processor
        SS2_B           (slot 2)        port 6, used to select the
        SS3_B           (slot 3)        current active slot

    5. other
        AC_B            (slot 3)        Input for external Organiser
                                        switch on



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4.3  POWER RAILS



A detailed description of the power supply circuitry is included in chapter
3.   The  main  properties  of  the  externally  available  power rails are
included here for reference.



     1.  Vb             (slot 3 only)

              This is the main Organiser system power rail, and is  fed  by
         the   Organiser  battery  via  a  forward  diode.   All  Organiser
         regulated power rails are derived from this rail.  Vb can be  used
         as  a  power  output  or  as  an external power input.  As a power
         output, the battery voltage minus a diode drop will appear at this
         pin (5.5 to 8.5 volts dependant on battery condition).  As a power
         input, the voltage applied  should  be  higher  than  the  battery
         voltage  to  ensure  no  current  drain  from  the battery.  It is
         recommended that an external power source also feeds  the  Vb  pin
         through  a  forward  diode,  to  ensure  no reverse current to the
         external source when it is powered  off.   In  this  configuration
         power  for the system is drawn from either the internal battery or
         the external power  source,  whichever  is  supplying  the  higher
         voltage.   Vb should be between 5.5 and 11.0 volts under a maximum
         system load of 175 mA.  The  lower  limit  is  determined  by  the
         dropout  voltage  of  the  5 volt pass regulators (the low battery
         indicator is triggered at approx.  5.3 volts on  Vb).   The  upper
         limit is defined primarily by the Vcc3 pass regulator - see below.

     2.  Vcc3           (all slots)

              This is the main power rail to the slots, and is regulated to
         5  volts  +/-5%.   It  is  derived  from  the  Vb rail above.  The
         regulator is a low-dropout type with a PNP pass  transistor  rated
         at  1  watt.   At  a Vb voltage of 11 volts the maximum DC current
         capacity of Vcc3 is therefore 167  mA  (167*(11-5)=1000  mW).   In
         practice  150  mA  should  be  used  as  the  rating of this rail,
         remembering that all three slots are  powered  in  parallel.   The
         power  budget  allocated  to each slot is 40 mA for an idle device
         and 70 mA for an active selected device.  Only one slot should  be
         active  at  any one time, giving 40+40+70=150 mA as the peak power
         drain with three devices present and one active.

              Vcc3 is switched on and off by the PACON_B  signal  from  the
         processor  port  6,  bit 7.  When this bit is defined as input the
         PACON_B signal is pulled high to leave the regulator  in  the  off
         state.   (When  off,  only  leakage  current  of  a few nA will be
         supplied to the Vcc3 rail).  To switch  Vcc3  on,  port  6  bit  7
         should be defined as output and low (0).

     3.  Vpp            (slots 1,2)

              This  rail  is  designed  specifically  for  programming   of
         datapack eproms, and may assume one of three voltages :-

         1.  0 volts            when Vcc3 is off

         2.  4.5 volts          when Vcc3 is on (diode drop below Vcc3)

         3.  21 volts +/-2%     when the 21 volt regulator is on


The 21 volt state is normally used in a pulsed mode under software control,
for  programming eproms with defined algorithms.  This is discussed further
in the Datapack section of the manual.



     ___________________________
4.4  DATA BUS (PROCESSOR PORT 2)



The data bus SD0-SD7 is an 8 bit bi-directional bus to all three slots, and
is  controlled  from  the  processor I/O port 2.  The notes below summarise
port 2 operation in the context of the Organiser system.

     1.  The primary use of port 2 is as an eight bit  parallel  I/O  port.
         Two registers control this function :-

             Port 2 data register       $0003

             Port 2 DDR                 $0001

         The DDR determines the I/O direction  of  the  port  bits  (0  for
         input, 1 for output).  Only 2 bits of the DDR are active :-

             Bit 0 defines the direction of SD0

             Bit 1 defines the direction of SD1-SD7

         The  DDR  is  a   write-only   register,   and   read-modify-write
         instructions should be used with caution.

     2.  With the DDR set to input, data present on the  bus  can  be  read
         through  the  data  register.   If no slot is active a $00 will be
         returned, defined by the eight pull-down  resistors  on  the  data
         lines.

         When the Organiser is off (processor in standby mode) the  DDR  is
         automatically  set  to  input, and remains in this state on system
         initialisation.  In subsequent operation this should  be  used  as
         the  rest  state,  and in particular should always be set to input
         whenever Vcc3 is switched off.

     3.  With the DDR set to output, data can be set  onto  the  bus  by  a
         write  to  the  data register.  Data is latched into the register,
         and will remain on the bus until a further write.  Note that  data
         can be written to the data register with the DDR set to input, and
         this data will be set onto the bus when the DDR is turned round.

     4.  Control of the bus and bus direction is  entirely  under  software
         control.   Control of devices in the slots is described further in
         the next section, but it is important to stress here that  control
         of  the port 2 DDR is vital for proper bus operation.  A condition
         where the DDR  is  set  to  output  and  a  slot  device  is  also
         outputting  to  the  bus  should  not  be  allowed to occur if bus
         contention and possible device damage are to be avoided.

     5.  In addition to the data bit I/O function, each bit of port 2 has a
         secondary  function  which may be selected under software control.
         When selected, the relevant bits assume their secondary  function,
         overriding   the  DDR  setting  where  necessary.   The  secondary
         functions are described in the processor manual.   An  example  of
         their  use  is  the  Organiser  RS232  interface,  which  uses the
         internal serial communications interface and the port 2 Tx and  Rx
         bits.   Note  that  in special cases such as this, various bits of
         the data bus may separately  be  defined  as  inputs  and  outputs
         simultaneously.




     ________________________________
4.5  CONTROL LINES (PROCESSOR PORT 6)

     Port 6 of the processor is  an  8  bit  I/O  port  controlled  by  two
registers :-

         Port 6 data register   $0017

         Port 6 DDR             $0016


     The DDR determines the direction of the port bits (0 for input, 1  for
output).  Each bit of the DDR determines the direction of the corresponding
bit of  the  data  register.   The  DDR  is  a  write  only  register,  and
read-modify-write instructions should be used with caution.

     When the Organiser is off (processor in  standby  mode),  the  DDR  is
automatically   set   to   input  and  remains  in  this  state  on  system
initialisation.  In this case the lines from the ports will take up  states
defined by the relevant external pull-up and pull-down resistors.

     The port bits are defined as follows :-
        bit 7   PACON_B

                This bit is used to switch the main Vcc3 power rail to the
                slots as described in section 4.3.

        bit 6   SS3_B
        bit 5   SS2_B
        bit 4   SS1_B

                These three bits are used to select the current active
                slot. The rest state should be with all three bits set high
                i.e. with all slots inactive. Note that when the relevant
                port bits are set to input, these lines are pulled high by
                external 6k8 ohm pull up resistors to the Vcc1 voltage
                rail. Vcc1 is the supply rail to the processor board and is
                present at all times (including when the Organiser is off).
                Of particular importance, these lines are pulled high
                whether or not the Vcc3 rail is on. This has been designed
                so that a small amount of power - of the order of 10
                micro-amps -can be drawn by each of the slots through the
                slot select line when the slots are otherwise powered down.
                Rampacks are an example of the use of this facility. To
                protect against unwanted power drain through this line, a
                blocking diode or transistor are normally employed in each
                device between the slot select input and the device
                circuit. A slot is selected by setting a "0" onto one of
                the three lines. Only one should be selected at any one
                time, and the software is responsible for ensuring this.
                Each device should be designed so that it can only output
                to the bus when its slot select line is pulled low.

        bit 3   SOE_B
        bit 2   SPGM_B
        bit 1   SMR
        bit 0   SCK

                These are four general purpose control lines used to
                control devices in the slots. All four are wired to slots 1
                and 2, but the SPGM_B signal is not available on slot 3.
                With the port bits defined as input, the rest state of all
                except SPGM_B is low. SPGM_B is pulled to the Vcc3 rail via
                a resistor, and so will be low with Vcc3 off and high with
                Vcc3 on.
                The way these lines are used is to some extent dependent
                on the type of device currently selected. They are normally
                used as outputs to control devices, but under special
                circumstances one or more of the lines may be defined as
                input. The four signal names are related to the functions
                of the lines when used to control 8 or 16k datapacks :-

                 SOE_B  directly controls the datapack eprom OE_B  signal
                 SPGM_B directly controls the datapack eprom PGM_B signal
                 SMR    resets the datapack address counters
                 SCK    clocks the datapack address counters

                These meanings are not fixed and the lines can be used in
                different ways depending on the particular active device.




     __________
4.6  AC_B INPUT

     The AC_B signal from slot 3 can be used by a top slot device to switch
the  Organiser  on.   Its function is the same as pressing the Organiser AC
key, but it is only effective when the machine is off.

     To activate this function, the AC_B signal should be pulled low by the
external  device,  by  an  open-collector  npn  transistor  or  other means
(internally the signal is pulled up to Vcc1 by a 47k  ohm  resistor).   The
RS232 interface is an example of the use of this.

     If the Organiser is off, pulling the AC_B line low will switch it  on.
When the Organiser is on the AC_B signal is disconnected and has no effect.
Note that if the line is pulled low permanently the Organiser will re-start
whenever it tries to switch off.