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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.


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:

power rails

0 volts

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)
data bus SD0
all slots 8 bit data bus from processor port 2
control bus SCK all slots 4 general control lines from processor port 6
SPGM_B slots 1,2
slot selection SS1_B slot 1 3 control lines from processor port 6, used to select the current active slot
SS2_B slot 2
SS3_B slot 3
other AC_B slot 3 input for external switch on

see Datapack Connector for pinout.


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

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 dependent 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.

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).

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.


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.

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.

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.

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.

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.

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.


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 above.
bit 6

bit 5

bit 4



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

bit 2

bit 1

bit 0




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.


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 restart whenever it tries to switch off.

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