Parallel Interface - Bus


PI-Bus [Parallel Interface - Bus].

PI-Bus is a synchronous, multidrop bus that supports digital message communications between up to 32 modules residing on a single backplane. The interface Datum size is 16 bits (single word) or 32 bits (double word). Messages can be routed to particular modules using either logical or physical addressing. The protocol specifies a set of bus state transitions to control the communications sequence and allow operation at maximum clock rate. Signal line and sequence error detection capability is incorporated into the bus definition with an optional single line error correction.

SAE-AS4710; PI-Bus {Parallel Interface - Bus}, released in 1996 is similar to the VME bus backplane structure, but uses BTL logic as the electrical interface. PI-Bus is an avionics fault-tolerance bus.
PI-Bus is also called the Parallel Intermodule Bus. PI-Bus provides a 32 bit wide data bus, with a 50M Bytes per second transfer rate, and uses a preemptive, priority-based protocol

Integrated Circuits:
Federal Stock Drawing Number; 5962-92317: Microcircuit, Digital, CMOS, Octal PI-Bus Transceiver, Monolithic Silicon.
Generic part number; DS1776 which is produced by National Semiconductor.

Supply voltage range (VCC) .... +4.5 V dc to +5.5 V dc, however the supply lines are not shown in the diagram.
Data lines on the PI-Bus are pulled high, or terminated, by a 30 to 40 ohm resistor to a voltage between 1.9 and 2.1 volts.
A 40 ohm resistor would be used with a lightly loaded bus or backplane, while a 30 ohm resistor is used for a fully loaded bus.

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PI-Bus Transceiver schematic
PI-Bus Transceiver IC

The DS1776 is an Octal bidirectional transceiver, but only the A to B path is latched.
The B side outputs are Open Collector with a Schottky diode in series with the output line, which is not common.
The A side outputs may be Tri-State using the OEA line.
The IC supports Wired-OR connections.

Main page; Avionics Data Buses
Note the DS1776 was released in 1996, and may be used driving the PI-Bus on the F22 Raptor; Advanced Tactical Fighter.

Main page; Protocol Analyzer Manufacturers.

Main page; COTS Card Manufacturers, all formats.

Editor note; I have no data on NATO or STANAG usage, or if the PI-Bus is used in civilian avionics or aviation.
The PI Bus may only be a US Military Avionic Bus; however no Military Specification has been found.
The Physical Interface implementation of PI Bus is a VME 'like' backplane, and the electrical interface is defined with the DS1776 IC.

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Modified 2/4/12
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