I have a small raspberry pi like gadget running Linux. I'm using uModbus to let it act as a Modbus Server so that it can provide data and control points to a larger PLC system.
My device is a gateway to a series of sub nodes. Let's pretend for example, they are micro weather data sensors. For each one, the device gathers pressure, humidity, temperature, and windspeed. The subnode count is variable but can be as many as 100. It is possible that at some point, there might be an additional value or two added to what the sensors can read.
To flatten these values into modbus registers, I can choose to order by subnode first or subnode second.
A subnode first scheme might look something like
baseAddress = 01000 + (subnodeIndex * 10)
And then the individual values per subnode are just offset from that.
The subnode second scheme would assign a baseAddress for each value type (e.g. temperature starts at 00100, humidity at 00300, etc).
Is there a strong reason to prefer one scheme over the other? Or is there a scheme I'm missing? It seems that there's value in banking values together to take advantage of single block reads, but at the same time, leaving space for future expansion encourages some form of padding between values.
Related
In the book Operating System Concepts by Silberschatz et al. and in other representations of paging, each Page Table (PT) entry is shown to contain a Frame Number(FN). I have been trying to solve some problems on this topic and the solutions also assume the same.
I am new to this topic and all I've read is that the PT (or part of it) is usually stored in the physical memory. This means that the PT must start at some particular physical address. This value is stored in the PTBR. So when the CPU generates a logical address which is divided into a Page Number(PN) and the offset, the PN must be compared against it's value in the PT to determine the corresponding FN, right? Suppose, p=14. Now, 14, in its binary form, must be checked in the PT to find the entry 14.
So, does this mean that some bits of each entry of the PT store the PN and the rest of them store the corresponding FN? Then what about the problems which calculate the PT size assuming that it's only a multiple of the number of bits of the FN and completely ignore the PN bits? Or is it that since the values in the PT are stored in increasing order of PN, some calculations are performed on the PTBR to get the particular PT entry? If so, once the calculation to access the particular PT entry has been done, how is the memory access switched from the base address in the PTBR to the 'n'th entry? What hardware is used for that switch?
Basically, what I'd like to know is, where is the index, p(page number), which checks for the corresponding FN stored? How exactly is this value checked against the PT entries to get the FN?
Physical Page Frames are stored sequentially in hardware.
A logical address consists of a logical page number and an offset into the page. For page translation, we can ignore the offset.
In the simplest case, the page table is an array of Page Table Entries. The Logical Page Number serves as an indicate into this table.
The Page Table Entry may or may not have a value for a Physical Page Frame.
This means that the PT must start at some particular physical address.
The address and size of the Page Table is usually specified by hardware registers.
So when the CPU generates a logical address which is divided into a Page Number(PN) and the offset, the PN must be compared against it's value in the PT to determine the corresponding FN, right?
You appear to be describing an INVERTED PAGE TABLE that is used on a few processors. In most systems the Page Number is an index into the table. There is no comparison.
I'm working on neighbor discovery protocols in wireless ad-hoc networks. There are many protocols that rely only on beacon messages between nodes when the discovery phase is going on. On the other hand, there are other approaches that try to transmit more information (like a node's neighbor table) during the discovery, in order to accelerate it. Depending on the time needed to listen to those messages the discovery latency and power consumption varies. Suppose that the same hardware is used to transmit them and that there aren't collisions.
I read that beacons can be sent extremely fast (less than 1ms easily) but I haven't found anything about how long it takes to send/receive a bigger message. Let say a message carrying around 50-500 numbers representing all the info about your neighbors. How much extra power is needed?
Update
Can this bigger message be divided into a bunch of beacon size messages? If it does, then I suppose the power used to transmit/listen grows linearly.
One possible solution is to divide the transmission in N different beacon-like messages with a small extra information to be able to put them back together. In this way, the power used grows linearly as N grows.
I have a project that consisted of transmitting data wirelessly from 15 tractors to a station, the maximum distance between tractor and station is 13 miles. I used a raspberry pi 3 to collect data from tractors. with some research I found that there is no wifi or GSM coverage so the only solution is to use RF communication using VHF. so is that possible with raspberry pi or I must add a modem? if yes, what is the criterion for choosing a modem? and please if you have any other information tell me?
and thank you for your time.
I had a similar issue but possibly a little more complex. I needed to cover a maximum distance of 22 kilometres and I wanted to monitor over 100 resources ranging from breeding stock to fences and gates etc. I too had no GSM access plus no direct line of sight access as the area is hilly and the breeders like the deep valleys. The solution I used was to make my own radio network using cheap radio repeaters. Everything was battery operated and was driven by the receivers powering up the transmitters. This means that the units consume only 40 micro amps on standby and when the transmitters transmit, in my case they consume around 100 to 200 milliamps.
In the house I have a little program that transmits a poll to the receivers every so often and waits for the units to reply. This gives me a big advantage because I can, via the repeater trail (as each repeater, the signal goes through, adds its code to the returning message) actually determine were my stock are.
Now for the big issue, how long do the batteries last? Well each unit has a 18650 battery. For the fence and gate controls this is charged by a small 5 volt solar panel and after 2 years running time I have not changed any of them. For the cattle units the length of time between charges depends solely on how often you poll the units (note each unit has its own code) with one exception (a bull who wants to roam and is a real escape artist) I only poll them once or twice a day and I swap the battery every two weeks.
The frequency I use is 433Mhz and the radio transmitters and receivers are very cheap ( less then 10 cents a pair if you by them in Australia) with a very small Attiny (I think) arduino per unit (around 30 cents each) and a length on wire (34.6cm long as an aerial) for the cattle and 69.2cm for the repeaters. Note these calculations are based on the frequency used i.e. 433Mhz.
As I had to install lots of the repeaters I contacted an organisation in China (sorry they no longer exist) and they created a tiny waterproof and rugged capsule that contained everything, while also improving on the design (range wise while reducing power) at a cost of $220 for 100 units not including batterys. I bought one lot as a test and now between myself and my neighbours we bought another 2000 units for only $2750.
In my case this was paid for in less then three months when during calving season I knew exactly were they were calving and was on site to assist. The first time I used it we saved a mother who was having a real issue.
To end this long message I am not an expert but I had an idea and hired people who were and the repeater approach certainly works over long distances and large areas (42 square kilometres).
Following on from the comments above, I'm not sure where you are located but spectrum around the 400mhz range is licensed in many countries so it would be worth checking exactly what you can use.
If this is your target then this is UHF rather than VHF so if you search for 'Raspberry PI UHF shield' or 'Raspberry PI UHF module' you will find some examples of cheap hardware you can add to your raspberry pi to support communication over these frequencies. Most of the results should include some software examples also.
There are also articles on using the pins on the PI to transmit directly by modulating the voltage them - this is almost certainly going to interfere with other communications so I doubt it would meet your needs.
I'm making a simple electric circuit simulator. It will (at least initially) only feature batteries, wires and resistors in series and parallel. However, I'm at a loss how best to simulate said circuit in a good way.
Specifically, I will have batteries and resistors with two contact points each, and wires that go between two contact points. I assume that each component will have a field for its resistance, the current through it and the voltage across it (current and voltage will, of course, be signed). Each component is given a resistance, and the batteries are given a voltage. The goal of the simulation is to assign correct values to all the other fields in real time as the player connects and disconnects components and wires.
These are the requirements:
It must be correct, including Ohm's and Kirchhoff's laws (I'm modeling real world circuits, and there is little point if the model does something completely different)
It must be numerically stable (we can't have uncontrolled oscillations or something just because two neighbouring resistors can't make up their minds together)
It should stabilize relatively quickly for, let's say, fewer than 30 components (having to wait a few seconds before the values are correct doesn't really satisfy "real time", but I really don't plan on using it for more than 10 or maybe 20 components)
The optimal formulation for me (how I envision this in my head) would be if I could assign a script to each component that took care of that component only, possibly by communicating field values with neighbouring components, and each component script works in parallel and adjusts as is needed
I only see problems here and no solutions. The biggest problem, I think, is Kirchhoff's voltage law (going around any sub-circuit, the voltage across all components, including signs, add up to 0), because that's a global law (it says somehting about a whole circuit and not just a single component / connection point). There is a mathematical reformulation saying that there exists a potential function on the points in the circuit (for instance, the voltage measured against the + pole of the battery), which is a bit more local, but I still don't see how to let a component know how much the voltage / potential drops across it.
Kirchhoff's current law (the net current flow into an intersection is 0) might also be trouble. It seems to force me to make intersections into separate objects to enforce it. I originally thought that I could just let each component have two lists (a left list and a right list) containing every other component that is connected to it at that point, but that might not make KCL easily enforcable.
I know there are circuit simulators out there, and they must have solved this exact problem somehow. I just can't find an explanation because if I try googling it, I only find the already made simulators and no explanations anywhere.
Question:
Design a distributed system to response the clients' query about "finding something within X miles".
If X is infinite, get all the "something" in the world (if they are all stored in your database)
You can think about two approaches:
when the number of potential results is small and number of queries big divide the space of coordinates between available machines and send queries only to machines which are responsible for areas which intersect with X-mile circle
when the number of potential results is big store objects dispersed, so that they are uniformly distributed on all machines (you can choose the machine by randomization or by objects' origin - it depends) and post every query to all machines and merge received results.
Further changes depend on getting more information about the problem nature.