Hardware Overview

The Psion SIBO serial protocol

The Psion SIBO serial protocol is a general purpose method of bi-directional serial data transfer. It has been designed for synchronous communication between a host controlling device and a number of slave devices. On a hardware level, the SIBO serial protocol is implemented through Psion ASICs. The controlling device must contain an ASIC2 (or ASIC9) and the slave devices an ASIC4 or ASIC5. The various Psion ASICs are described in more detail below.

The synchronous SIBO serial protocol interface consists of 2 wires:

  • CLK - A clock output from the controller to the slaves. Nominally 3.84 MHz.
  • DATA - A bi-directional synchronous data line.

The data is transferred using a series of 12 bit frames including 8 data bits each. This equates to a theoretical maximum data transfer rate of approximately 312 Kbytes/second. Other bits of the frame contain control information. The "system" is generically defined by 2 protocol layers, namely the Physical layer and the Transport layer. These layers are described in detail in the next chapter.

Psion ASICs and what they do

ASIC stands for Application Specific Integrated Circuit and as previously indicated, these devices are widely used within Psion hardware. Summaries of the functionality of each ASIC that is relevant to peripheral development are presented below:


ASIC1 is the main system controller chip for the SIBO architecture. It connects directly to the 8086-based processor (i.e. the V30H) controlling all bus cycles to and from the processor. This configuration effectively forms a micro-controller like device that executes 8086 instruction codes. ASIC 1 is made up of a number of functional blocks including a bus controller, a programmable timer, an eight input interrupt controller, an LCD controller and the memory decoding circuitry.


ASIC2 is the peripheral controller chip for the SIBO architecture. It contains the system clock oscillator and controls switching between the standby and operating states. ASIC2 provides an interface to the power supply, keyboard, buzzer and SSDs. ASIC 2 includes the eight-channel SIBO serial protocol controller and provides interface circuitry to both the reduced external and extended internal peripheral expansion ports.


ASIC4 is a serial protocol slave IC for addressing memory and general memory-mapped peripherals. It is used in SSDs to convert SIBO serial protocol signals into addresses within the memory range of the memory pack. ASIC4 was designed to be a cut-down version of ASIC5 which was the original SIBO serial protocol slave chip.


ASIC5 is a general purpose I/O chip with a built-in UART that can be set to run in a number of different modes thereby simplifying the task of peripheral design. For example, it is possible to set up ASIC5 to run as a Centronics parallel port interface, an 8-bit parallel I/O port, a serial bar code controller or a serial RS232 converter.


ASIC9 is a composite chip comprising of a V30H processor, ASIC1, ASIC2 and general I/O and PSU control logic all on one IC. ASIC9 thus integrates all the digital logic required to produce a SIBO architecture computer less the memory onto one chip. ASIC9 has a few additional features such as an extra free-running clock (FRC) and a codec interface for sound.


Psion peripherals usually incorporate some circuitry to generate hardware interrupts. Both reduced external expansion ports (such as the LIF connector on the Workabout or the 6-pin serial port connector on the S3a) and extended internal expansion ports (such as the two single row 25-way HC connectors) carry an interrupt line. This is an active high input to the host machine's interrupt controller circuitry which resides on the logical equivalent of ASIC1. The OS intercepts all interrupts and can be requested to call a particular function within a controlling device driver. Eight hardware interrupts are supported by SIBO hardware. IRQ0 is the highest priority and IRQ7 the lowest. All interrupts are level triggered and must be serviced in the following order:

  1. Device asserts the appropriate interrupt request line.
  2. The interrupt controller unit within either ASIC2 or ASIC9 places onto the data bus the vector of the highest priority device with an interrupt pending. This enables the CPU to jump to the correct interrupt service routine code.
  3. During the interrupt service routine, the software clears the interrupt line by some action specific to the device.
  4. The interrupt service routine then informs the interrupt controller that the interrupt has been cleared by writing to the non-specific end of interrupt (NSEOI) location.
  5. If another interrupt is pending then go back to the second step.

With 8086-based processors, it is not possible to have nested interrupts.

The current range of Psion peripherals

There are currently a number of Psion peripherals in use and some of the key ones are outlined below in order to provide the developer with a feel for peripheral design issues:


Solid State Disks (or Memory Packs) use a built-in ASIC4 to decode SIBO serial protocol signals into memory addresses within the memory range of the SSD.

The HC Printer

The HC Printer translates SIBO serial protocol signals transmitted across the single row 25-way extended internal expansion socket of the host HC into a parallel 8-bit format that is compatible with the universal Centronics printer interface standard. The HC Printer unit contains an ASIC5 running in Centronics interface mode which acts as the serial protocol slave and requires a small number of support chips.

Psion 3-Fax

The 3-Fax contains an ASIC4 and a memory-mapped modem chip set which permits the host machine to transmit (but not receive) fax messages.


The Psion Barcode reader employs an ASIC5 running in serial mode to read the data received from the barcode decoder chip into a SIBO serial protocol format that can be transmitted to the host ASIC2/ASIC9.

Workabout RS232 Interface

This peripheral connects to the single row 26-way extended internal expansion port of the Workabout. It incorporates an ASIC5 running in its default mode to translate SIBO serial protocol signals into a TTL level (+/-5v) serial RS232 format using ASIC5's on-board UART. The TTL level RS232 signals are converted into the standard EIA format (+/-12v) before coming out on the conventional RS232 9-pin D-type connector.