Properly terminating the main data bus on each end makes the bus appear like an infinite line. Stub loading effects can be minimized on the bus by properly designed coupling. Coupling connects a terminal device to the main data bus via interconnecting buses called stubs. Connecting terminals creates a load on the main bus, creating impedance mismatches and resultant signal reflections that can distort and degrade signal quality on the bus.
System error rate performance and good signal to noise ratio require a balance between stub impedance being low enough to provide adequate terminal signal levels but high enough to reduce signal distortion from reflections. Direct coupled connections are wired directly to the bus cabling. The isolation resistors and transformer are internal to the terminal device, not requiring additional coupling hardware.
Direct coupled connections can only be used with stub lengths of less than one foot. Isolation resistors provide some protection for the main bus in the event of a stub or terminal short, but MIL-STDB cautions the use of direct coupling because a terminal short could disable the entire bus. Direct stubs can also cause significant impedance mismatches on the bus. Transformer coupling utilizes a second isolation transformer, located external to the terminal device, in its own housing with the isolation resistors.
Transformer coupling extends the stub length to 20 feet and provides electrical isolation, better impedance matching and higher noise rejection characteristics than direct coupling. The electrical isolation prevents a terminal fault or stub impedance mismatch from affecting bus performance. Bus topology refers to the physical layout and connections of each device attached to the data bus. Single level topologies are the most basic, easy to design and widely implemented layouts with all terminal devices connected to a single bus.
When an error is detected, the remote terminal must suppress transmission of the status word. The Instrumentation bit 10 differentiates a command word form a status word both have the same sync pattern. Since this bit is the most significant bit of the subaddress field, using it as an instrumentation bit reduces the number of available subaddresses from 30 to Because of this limitation, most systems today use techniques other than the instrumentation bit to differentiate between command and status words.
The Service Request bit 11 enables a terminal to inform the bus controller that it needs to be serviced. It is typically used when the bus controller is polling the terminals. This bit is typically not used in modern system designs and is discouraged by Notice 2 of the standard. After setting this bit, the remote terminal becomes the bus controller. A Remote Terminal can be used as an interface between the bus es and a subsystem or as a connector between this bus and another bus.
A subsystem is the sender or user of the information transferred on the bus. A remote terminal contains all the components needed to transfer the data from the sender source to the destination user. The Bus Controller BC manages the flow of data on the buses. Only one bus controller can be active at a given time. A Word controller, which handles one word at a time, is seldom used today because of the processing burden it places on the subsystem. A Message Controller handles one message at a time, interacting with the computer only when a message is complete or when a fault occurs.
A Frame Controller can process multiple message in a defined sequence, interrupting the computer only when the message stream is complete or after an error is detected. A Bus Monitor is not able to transmit messages on the bus; its function is to monitor, and record, messages being transmitted on the bus without disrupting other devices.
A bus monitor can be set up to record selected subsets of the messages on the bus. It can also be set up as a backup bus controller, ready to take over whenever needed.
Each IO board has 2 channels. Each channel is a complete system with Side A and Side B. UEI has a wide variety of solutions for your defense and aerospace applications.
Please click on the below to learn more. The major minor frame scheduler is designed to run cyclic, multi-rate commands over and over again at preselected frequencies. Sometimes you need to send a command, or series of commands, just once. We've developed a technique to help you accomplish this. There are different types of bus controllers in the UEI system. In this video we show you how to identify each one and why you would use one or the other. Need to record data? In this brief tutorial we demonstrate the major concepts behind the UEI major and minor frame scheduler.
We have a simple solution. In addition, the simulated bus networks will deliver approximately the same stub voltages that will be received on a compliant bus, assuming a short bus with relatively low cable attenuation. That is, about 7 volts peak-to-peak to the stubs for direct-coupled receiving terminals, and about 5 volts peak-to-peak to the stubs for transformer-coupled receiving terminals.
Question submitted August 10, A. Bus monitors are commonly used in production flight systems. Bus monitors may be embedded into BCs or RTs, or can be implemented as standalone monitors. By definition, standalone bus monitors never transmit on a bus. In operation, an RT without an embedded monitor stores only command and data words sent to its own RT address.
A standalone bus monitor can be programmed to store all words received over a bus. Question submitted August 5, A. By so doing, the decoder will make signal polarity determinations on each Manchester half-bit and is able to make timing adjustments during the decoding of each received Manchester half-bit, data bit, and word. Question submitted July 24, A. Paragraph 4. However in others, twinax cable refers to two signal wires running in parallel not twisted , enclosed with a shield. Question submitted July 9, A.
The reflection coefficients encountered at each stub on the bus will be different for the fundamental and each harmonic. In addition to cable attenuation, each stub on the bus will attenuate the signal. Since the stub impedance decreases with increasing frequency, the amplitudes of the signal harmonics will be attenuated more than the amplitude of the fundamental signal components kHz, kHz, kHz, or 1 MHz at each stub.
Question submitted July 6, A. If this is done, there must be some defined method for switching the operation of individual RTs between inactive and active. This may be based on the reception of specific messages.
This includes monitoring for high bit counts for all transmitted words, and high word counts for all transmitted messages. After it switches buses, it should send a Transmitter shutdown mode code message to the babbling RT on the alternate bus in order to force it to shut down its transmitter on the original bus. Question submitted June 24, A.
While this operation for bus monitors is not defined or required by MIL-STD, it is something that certainly may be implemented by a bus monitor. Question submitted June 10, A.
For many applications, the message formatting for specific parameters is defined on a system basis by means of an Interface Control Document ICD.
The precise operation of the Terminal Flag status word bit is not defined by the standard. A third possibility is that the Terminal Flag may become set to indicate that the RT failed its most recent commanded internal self-test.
Question submitted May 19, A. If there were two active bus controllers on the same bus, the likely effect is that there will be a high number of collisions on the bus, resulting in a large number of errors. Normally at any given time, there is one active bus controller and one or more backup bus controllers on a bus.
The standard does not call out specific means for dealing with the situation of two simultaneously active bus controllers. For this, it is the responsibility of the systemdesigner to reconcile this at a higher level.
One possibility is for all bus controllers to also include built-in bus monitors that operate independently of the BCs. This allows the active bus controller to intentionally transfer bus control to a different node on the bus. Question submitted May 18, A.
Question submitted May 8, A. According to MIL-STDB, RT response time is defined as the time from the mid-bit zero crossing for the parity bit of the last word received from the bus controller to the mid-sync zero crossing of the Status word sync from the responding RT. The terminal shall respond to an input signal whose peak-to-peak amplitude, line-to-line, is within the range of.
The terminal shall not respond to an input signal whose peak-to-peak amplitude, line-to-line, is within the range of 0. The voltages are measured at point A on figure 9. The problem with this definition for the first half of a Command word sync or a Status word sync is that it does not begin with a zero crossing. Moreover, from a practical standpoint, in most real-world implementations, the time starting from the departure from zero to the first zero crossing will be greater than ns.
MIL-STDB defines the interoperability point only at the boundary of the terminal; that is, the stub connection to the isolation transformer. Question submitted April 22, A. The driving reason for its development was to reduce the amount of wiring in military aircraft by connecting to various on-board systems over a common multi-drop data bus. The result was the elimination of literally miles of point-to-point wiring that was used to connect between different systems on older military aircraft.
The use of a multi-drop data bus also improved maintenance and logistics by enabling system upgrades to be made by updating software.
With the exception of certain mode code messages, paragraph 30 of MIL-STD Notice 2 see below forbids the transmission of broadcast messages by bus controllers. However, Remote terminals are permitted to implement broadcast. The only broadcast commands allowed to be transmitted on the data bus by the bus controller shall be the broadcast mode commands identified in table 1.
The broadcast option may be implemented in remote terminals. However, if implemented, the RT shall be capable of distinguishing between a broadcast and a non-broadcast message to the same subaddress for non-mode command messages. The RT address of is still reserved for broadcast and shall not be used for any other purpose.
Question submitted April 3, A. Specific message formats e. Alternatively, I suspect that certain customers are going to request other specific formats for GPS data. As for broadcast, with the exception of certain mode code messages, paragraph 30 of MIL-STD Notice 2 see below forbids the transmission of broadcast messages by bus controllers. Question submitted March 12, A. B loopback wires and maintain shielding continuity. Question submitted February 24, A.
Normally, no. For the case of the last length of bus prior to the terminator,. Assuming a voltage wave propagating from left to right in Figure 3, the bus voltage just after a stub connection is computed by: These calculations repeat back until the transmitting BC or RT terminal. Question submitted February 18, A.
Unused stubs should not be terminated. Both ends of the bus must be terminated in the bus characteristic impedance Z 0 , which is required to be in the range from 70 to 85 ohms. Switching from one bus to the other is the responsibility of the bus controller BC. When the BC observes multiple errors from the same RT, it needs to stop transmitting messages over the bus A or B resulting in the non-responses or faulty responses from that RT, and start transmitting messages to that RT over the alternate bus.
With modern BCs, this bus switching is typically done autonomously by the BC hardware. Question submitted February 2, A. Dual redundant RTs normally have two connectors, one for each stub connection, rather than four connectors the respective transmitter outputs and receiver inputs are normally connected together internally.
You will need to check the documentation on your RT. One possibility is that your system includes two dual redundant RTs, rather than one. In any case, to test a dual redundant RT, at any given time, the active transmitting bus controller channel should only be connected to one of the two stubs of a dual redundant RT. Question submitted January 15, A. Question submitted November 26, A. In most architectures, these words are written by the BC, RT, or Monitor during and after the processing of individual messages.
To determine the specific descriptor structures for the component or board that you are using, you will need to refer to the component or board documentation. Question submitted November 13, A. All voltages defined in peak-to-peak volts. These include the following:Direct-coupled transmitter output and bus voltage: 6 to 9 voltsDirect-coupled receiver stub voltage: 1. Question submitted October 30, A. Therefore from a practical standpoint, depending on the length of the bus cable, the stub impedance from adding more terminals on a bus will reduce the bus voltage.
Question submitted October 27, A. Paragraph Question submitted August 27, A. It is certainly possible to write a software application that conforms to DOB development practices and run that application in a compliant operating system such as Green Hills IntegrityB or VxWorks The BC can tell if it got a correct response, an incorrect response due to parity, word count, or bit count error, or no response.
This provides some information about the state of the RT, but has limited diagnostic utility, as for example, it is not possible to tell if an RT is unpowered, or has a parity error on its address inputs. It is up to the system designer to ensure that the system will behave appropriately in all potential cases.
Question submitted July 25, A. If the leakage paths to ground end up being balanced and there is no static charge on the line, then the signal will be balanced. If not there will be an apparent imbalance that will be filtered out by the isolation transformer on the receiver. Specifically, if you try to make a single-ended measurement of the voltage on one of the signal wires against ground with an oscilloscope, you could end up with any waveform, from 0v to a 28v swing, with a possible DC signal overlaid on top.
Any measurements of the signal on the bus or stubs should be taken differentially, though because fo the transformer coupling, it is acceptable to use a single-ended probe on the positive signal with the ground clip attached to the negative signal.
Question submitted June 13, A. Each terminal has a specific frequency or set of frequencies that it is able to support. The particular frequencies supported are design choices made by the terminal manufacturer, the specification only controls what appears on the bus.
I'm assuming that by master mode, you are referring to operation as a Bus Controller, and by slave mode you are referring to standard Remote Terminal operation. As long as the BC is set to idle mode and does not attempt to transmit anything on the bus until it has been configured by the host, then there will be no difficulty. Immediately after power-up, the terminal will look the same to the rest of the bus as it did when it was still powered down, and this will only change when it is programmed by the host in whatever fashion you determine, at which point it will behave as the correctly configured RT that it is.
Question submitted June 11, A. If the rugged and hermetic hybrid package is not required, both the Micro-Ace TE and Total-Ace can be configured to operate the same way. Both use standard BGA packages, with the Total-Ace offering the extra advantage of incorporating the isolation transformer to simplify the board design. Question submitted May 12, A. One way to accomplish that would be designate one subaddress as the data status and load it with the amount of data that the RT wants to send to the BC.
The RT could then poll it periodically to see if there is any data to fetch, and then read the data from another subaddress, repeating until the full amount has been transferred. Question submitted May 9, A. For that you will have to refer to the individual datasheets. The power dissipation of the bus as a whole will depend on how many terminals of what types are connected, how they are connected transformer- or direct-coupled as well as the lengths of each stub and the bus segments that connect them and the duty cycles at which they operate.
Therefore the system must be designed such that this is always true, or some other allowance must be made, such that stale data is acceptable, or the RT sets the busy flag to show that it cannot return at that time.
The use of the subsystem flag is not defined in the spec, so it could potentially be used to signal that the returned data is not valid and the BC should poll again. However, this would be a system specific usage and would have to be defined in the system specification. Question submitted April 26, A. MIL-STD initially supported 32 Remote terminal address in Revision A, but when Revision B was introduced, one of the changes was to add broadcast commands that can reach all remote terminals with a single message using the RT31 address.
This obviously means that the RT31 address was no longer available for normal use. Question submitted March 28, A. A superseding command occurs when an RT receives a command, but before it finishes responding, it receives another command on the same bus. In this case it is required to drop the first command and respond to the second. Yes, the bus will continue to function even if a stub is shorted because the isolation resistors in the coupler combined with the coupling transformer will provide an apparent impedance of 1.
This will still cause reflections, and if multiple stubs are shorted may cause enough attenuation to prevent successful operation of the remaining stubs. However, this is dependent not just on how many stubs are shorted, but where they are located with respect to each other and the terminals that are still operational.
That would allow you to define a sub-address as either a queuing port or a sample port. As a sample port, the RT will respond with the first N words from a single data buffer associated with that sub-address N is the word count from the command word.
If a new command is received before the RT updates the buffer than the same block of data will be sent again. As a queuing port, the RT will respond with the next N words from a queued data buffer associated with that subaddress. A queuing port allows the RT to pre-load the sub-address buffer with multiple messages worth of data. The RT will transmit the next N words. In either case, RT writes to the sub-address data buffer and bus controller reads transmit commands from the sub-address need some form of synchronization flow control.
A sample port configuration may transmit the same block of data twice if the RT does not provide the next block of data in time while a queuing port configuration may be requested to transmit beyond the end of the buffer. In addition error handling needs to be implemented.
An upper layer protocol or header word may be required. Question submitted October 28, A. Your RT should also have this connection so it can resume receiving commands from the Bus Controller. Please note that is a violation to send any commands simultaneously on both A and B busses. The purpose of Subaddresses is to further identify a specific data payload that is to be sent or received.
Typically, each within a Remote Terminal are subaddresses, defined to contain specific data. Requesting data from the a particular RT Address, but different subaddresses allows you to accesses different data parameters of the Remote Terminal. A common analogy is the marking on a postage letter.
The mailing address would be the RT Address. Question submitted October 3, A. Question submitted September 23, A. Question submitted September 30, A. The isolation transformer power dissipation while transmitting is basically the sum of the I2R losses for the primary and secondary windings. Question submitted September 17, A. In theory, this could happen. For example, in normal operation, the host will anticipate that the BC will issue an interrupt request periodically.
Alternatively, the host might poll the BC to determine the status of messages to be transmitted possibly based on a timer. In either case, the software should include mechanisms to detect when the BC has not processed the anticipated frame of scheduled messages. If such an error is detected, then the host should shut down and reset the BC.
Question submitted September 16, A. Two ways of performing this test are: 1 with the oscilloscope connected to both stub legs and performing a true differential measurement by inverting the signal from one of the two stub lengths and adding the two signals; and 2 with a single-ended measurement, provided that one leg of the stub is connected to the same ground as the oscilloscope.
For either method, you should also connect the cable shield to the same ground as the oscilloscope. Question submitted September 10, A. Question submitted August 6, A. Question submitted May 16, A. Question submitted May 14, A. Almost all buses have at least one remote terminal.
However at least in theory, the scenario that you describe, with a data recorder with a built-in Bus Monitor, is possible. Question submitted April 15, A. In addition to its simplicity, another advantage of Manchester encoding is its transition density.
Since Manchester provides a minimum of one signal transition per bit time, this helps to facilitate reliable clock recovery, and the use of oversampling decoding techniques. Further, Manchester encoding provides a balanced waveform with zero DC component, thereby enabling transformer isolation.
There is a 0. Z0 isolation resistor in series with each leg of the transformer, resulting in a total impedance of 2? Because of the 1. That is, nodes on the bus that act as either RTs, Monitors, or both simultaneously, and can switch over to operate as BC if the primary BC fails.
For example, a node on a bus can be functioning as RT and Monitor simultaneously. A bus Monitor captures all data transmitted by the BC and all RTs on a data bus, or possibly a filtered subset of all data. The time tag resolution is software programmable by means of bits 9, 8, and 7 of Configuration Register 2. In addition, the value of the bit Time Tag Register may be written or read at any time. If an intermittent failure occurs only during taxiing, then this would appear to point to a vibration-related problem, possibly having to do with connectors.
I would begin by putting a monitor on the bus. With a bus monitor, it should be relatively easy to determine precisely what is failing. That is, is the bus controller failing to send out messages according to its normal schedule? Or, is the BC sending out all messages on-schedule, but is a remote terminal more likely or multiple remote terminals less likely failing to respond, or transmitting invalid responses?
Once you determine the failing BC or RT, then the next step would be to try to isolate the precise fault. If the signals are all OK there, this would point to the connections to the respective bus coupler, or to the coupler itself. If the signals are faulty there, then this would more likely point to a problem with the BC or RT in the attached system box.
Question submitted March 1, A. As you point out, having two simultaneous streams on the same data bus will certainly result in a lot of data errors. As to whether or not this will result in damage to the transceivers on the data bus, in most cases, I suspect that they would not be damaged. During the times that the voltage is doubled, this could result in voltages in the range of 12 to 18 volts peak-to-peak on the data bus, or 8. Since these voltages are well below the typical absolute maximum ratings of typical transceivers, this should not result in damage to other terminals on the bus.
During the times that the two signals driving the bus are driving opposite polarities, the bus voltage will be approximately zero volts. This is equivalent to driving a shorted out data bus. The normal load seen by a transformer-coupled terminal is Z0, which is nominally 78 ohms.
If the interference by the other transmitter is additive half the time and subtractive half the time, then the increase in current and power dissipation will be about half of that, or about Of course, if there are periods when only one transmitter is driving the bus also highly likely , then this will reduce the increase in transmitter dissipation accordingly. Note that for a fault condition consisting of one shorted stub on a bus, if you run the numbers, you can see that the load seen by a transformer-coupled terminal connected to a different stub i.
This is less of a reduction in nominal load impedance than the case you described. Question submitted January 25, A. The characteristic impedance of the cable Z 0 is specified to be in the range from 70 to 85 ohms, with a nominal value of 78 ohms.
Each data bus is terminated at each end with a resistor of nominal value Z 0. There are connections at various points along the bus to individual terminals, with the connection to each terminal called a stub. There are two methods for connecting between the bus and a terminal, direct and transformer-coupled. A direct coupled connection involves a direct connection between the bus and the terminal, and is limited in length to a maximum recommended distance of 12 inches.
For a transformer-coupled connection, there are two protective resistors of value 0. The maximum recommended stub length between the coupling transformer and the terminal is 20 feet. There is a parity bit associated with each bit word transmitted over a bus. This to allow the receiving terminal to be able to check the validity of received words. The history and basics of the Milbus b The Mil-StdB or Milbus is a standard defining characteristics of a serial multiplex data bus.
Validation A number of Validation and Production Test Plan are available and defining a standard set of tests covering terminals and systems. Interfaces Mil Bus Stub to Unit Connector Each Mil-Std B interface basically will comprise three elements: the chip implementing the data link layer, the transceiver and the transformer.
Here are some examples of commercial connectors and interfaces of the Milbus. Mil Bus Typical Architecture. Mil Bus and Stub.
Mil Bus Coupler.
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