A Brief comparison between mil std 1553 and major commercial data bus systems
MIL STD 1553 is ideal for all real time mission critical applications, particularly in harsh environments. For civil system applications, ARINC 429 and BITBUS are commonly used.
Comparing MIL STD 1553 with commercial data bus systems is a complex task; however, we have tried our best to present the core differences between MIL-STD-1553 and ARINC 429 along with BITBUS in this article. Industrial buses use different terminologies and are compatible with open framework of standards; this makes objective assessment difficult and identification of similar systems almost impossible. BITBUS is used for communication of real time distributed control systems, whereas, ARINC 429, is the most commonly used bus system in civil aircraft applications.
Comparing MIL-STD-1553 and ARINC 429
ARINC 429 was chosen for comparison with MIL-STD-1553 as it is the primary data bus system used in the civil transport aircraft avionics and can help us understand why two data bus systems are too different despite being used on airborne platforms.
Following are the major differences between ARINC 429 and MIL-STD-1553:
(a) Terminal addressing is not used, instead data tagging is employed.
(b) ARINC 429 comes with permanent broadcast operational mode.
(c) Receiver and transmitter terminals are directly coupled.
(d) RZ bipolar signaling code is used in ARINC 429 as compared to Manchester bi-phase in MIL-STD-1553.
(e) Last and the most pertinent, ARINC 429 has a data rate of 100 kbit/s max as compared to 1 Mbit/s in MIL-STD-1553.
It is quite evident that ARINC 429 offers a much lower data bus speed which is designed to suffice the requirements of a more benign environment of commercial aircraft avionics systems. Furthermore, ARINC 429 does not require lesser number of interface electronics and bus controller, which makes it a simpler system as compared to MIL-STD-1553.
Comparing MIL STD 1553 and BITBUS
Intel Corporation developed BITBUS in 1983 and introduced it in the public domain describing it as an interconnect for the construction of real time distributed control systems. BITBUS comes in to variants, synchronous and self-clocked versions. Moreover repeater option is also available to allow for transmission over longer distances. In this article self-clocked mode without repeaters is considered for comparison.
The major similarity between BITBUS and MIL STD 1553 is that they both use twisted pair cable to communicate between multiple nodes (Max: 28), one of which is assigned the role of bus controller, which is the master node, and the rest of the nodes are slaves. It has the provision to assign any node as the Master node. MIL-STD-1553 bus access method is also time division multiplexed with communication facility provided between master and slaves.
However, at the physical level BITBUS differs from MIL-STD-1553, and the major differences are as follows.
(a) BITBUS data bus has a direct coupling with transceiver without any isolation. No transformer or other isolation is used.
(b) BITBUS uses NRZI signaling code as compared to Manchester bi-phase used by MIL-STD-1553.
(c) BITBUS uses RS485 standard transceivers, which have a lower signaling levels.
(d) BITBUS has a lower data rate (375 kbit/s Max) as compared to 1 Mbit/s offered by MIL-STD-1553.
All these differences contribute to a reduced performance and substandard fault protection in critical mission environments. The only fundamental advantage offered by BITBUS is that is cuts the systems complexity and costs by not using coupling transformers and used lesser number of interfaces and transceivers.
The main conclusion can be submitted here that BITBUS can compete with MIL-STD-1553, if less demanding industrial applications are considered as compared to ARINC 429; whereas for real time control in harsh environments and mission critical operations, MIL-STD-1553 is by far superior in both performance and reliability.