I have a Modbus server implemented with pymodbus. This server has a thread that update the internal registers to simulate a variable environment from the field. I need to update a file when I receive a frame containing a write function code. I tried to implement the CustomDataBlock as suggested here, but that's not exactly what I need: in this example, the code is called every time a value is changed, hence also in my "internal" updating writer function.
I want some code to be called only when my server receive a frame with writing function codes.
Any idea?
Thank you
I found the solution by myself:
inherit ModbusRtuFramer
overload processIncomingPacket function
Related
I found some example code online that I'm trying to use to do a CNAME record lookup (notice that I pass a callback block that I want to be run):
DNSServiceQueryRecord(serviceRef, 0, 0, domainName, UInt16(kDNSServiceType_CNAME), UInt16(kDNSServiceClass_IN), callback, &mutableCompletionHandler);
DNSServiceProcessResult(serviceRef.pointee)
The problem is that this code is getting blocked at DNSServiceProcessResult(serviceRef.pointee) and the callback is never called. According to Apple's documentation for DNSServiceProcessResult, I need to
Use DNSServiceRefSockFD in conjunction with a run loop or select() to determine the presence of a response from the server before calling this function to process the reply without blocking.
So I looked at DNSServiceRefSockFD and found that I could create a dnssd_sock_t with DNSServiceRefSockFD(serviceRef.pointee). But now that I have the socket, I'm not sure how to "use it in conjunction with a run loop" as an event source for the run loop (according to the DNSServiceRefSockFD documentation).
I'm just not understanding how this works. I don't understand how to use the dnsssd_sock_t as an event source to a run loop so that I can call DNSServiceProcessResult at the right time without blocking so that my callback will actually run.
If it's better to use the socket as a kqueue event source or in a select() loop (as the documentation mentions), I'm fine with that, but I don't know how to do that either.
CoreFoundation can be quite cryptic, so any help is much appreciated!
And if there's a better way to do a CNAME record lookup then, by all means, please share!
See my (ethan-gerardot) comments on https://gist.github.com/fikeminkel/a9c4bc4d0348527e8df3690e242038d3
The first paragraph answers how to get the callback to be called without blocking.
I have just started to work with PLCs and I need to modify parameters of a sinamics from a PLC. I know that they may have different accesible levels and that they are modified by functions. So the question is:
Can I the parameters be directly modified?
Or if it is not possible, how do I programm and include that functions?
At least try to find some information.
http://support.automation.siemens.com/WW/view/en/34677186/136000&cspltfrm=39&cssw=0&csbinh=8
Sinamics drives parameters can be modified from the plc code using _writedriveparameter function.
One thing to remember is: never use this function in two places at the same time. If you do it the function hangs and to restore the correct working it is necessary to reboot the cpu.
Also remember to check the function result to call the function in the correct way (result 7001,7002,7003 ecc). You have to check if the function is already working or not.
I am attempting to run MOOS-IvP on a Beaglebone Black
On attempting to run the MOOS database it continuously throws the exception
"Exception Thrown in listen loop: Error Listening To Socket. Operation not supported"
This software runs on a Raspberry Pi
Any ideas what might be the issue?
I have found the problem and fixed it.
When the socket is created it needs to be TCP. However when getprotobyname(_sName) is called in the XPCGetProtocol class to lookup the correct protocol number in /etc/protocols it returns the value of the previous time it was called, which was when a UDP socket was setup.
To fix it I simply called the function twice, the second time it returns the correct value.
I am not sure why it would return incorrect the first time but this works!
I also encountered this error while working with a BeagleBone Black running Ubuntu 14.04. However, the solution of running the request twice did not work. More troubleshooting led me to determine that the one that was supposed to be a TCP socket was opened after another process had opened a UDP socket. The structure returned by getprotobyname() is a pointer to a static location that does not change from call to call, but does get updated with the protocol details (see here although for another Unix os). Therefore, the second call by another process overwrites the original details.
This then gets tested during socket creation in the constructor of XPCSocket, and results in creation of a UDP socket where it should have been a TCP socket. This could probably be fixed by adding a lock to this function, but I took the non-blocking approach to initialize the requested protocol using the string the constructor was called with (_sProtocol) instead of the one returned in the socketProtocol structure. In addition, I modified the XPCGetProtocol class to store the protocol number in a member variable that would not be changed upon subsequent calls to getprotobyname().
My modifications can be found here.
In the past, I have used libraries that would allow me to register a callback so that the library can call my method when some event happens (e.g. it is common to see in code that use GUI libraries to look like button.onClick(clickHandler)).
Naively, I suppose the library's handling mechanism could be implemented like:
while(1){
if (event1) { event1Handler(); }
if (event2) { event2Handler(); }
...
}
but that would be really wasteful right? Or is that really how it is done (for instance do well known GUI libraries like java swing, or GTK+ do it this way)?
background:
This question hadn't really occured to me until I encountered curses. I thought about implementing my own callback system, until I realized I didn't know how.
The while loop will typically wait for an interrupt from the user (GetMessage in Windows). When an interrupt arrives GetMessage returns and then it ends up in the callback function. The if statements are typically implemented as a switch-case. See Windows Message Loop on Wikipedia.
In more detail, what happens is the following:
The user application calls GetMessage, which forces the process to sleep until an input message for that application arrives from the systems queue. When a message arrives, the user app calls DispatchMessage, which calls the callback function associated with the window that the message was aimed at.
Windows API uses one callback which handles all events in a switch case. Other libraries use one callback per event class instead.
The function pointers themselves are stored together with other window data in a struct.
Callback system implementation probably has different implementation in different technologies, however, I suppose they should be working this way:
A data structure stores the callback IDs and pointers to the handlers.
A callback handler has a validator
Event handlers have callback callers, which know what are the possible callbacks and check their validity this way:
for each callback in event.callbacks
if (callback.isValid())
call callback()
end if
end for
When you add a handler to a function the system will automatically know where the callback is valid and will add the callback to the datastructure described in 1.
Correct me if I'm wrong, this description is just a guess.
Is it a function?
Is it a function being called from the source?
Or, is it a function being returned from the destination?
Or, is it just executing a function at the destination?
Or, is it a value returned from a function passed to the destination?
A callback is the building block of asynchronous processing.
Think of it this way: when you call someone and they don't answer, you leave a message and your phone number. Later on, the person calls you back based on the phone number you left.
A callback works in a similar manner.
You ask an API for a long running operation and you provide a method from within your code to be called with the result of the operation. The API does its work and when the result is ready, it calls your callback method.
From the great Wikipedia:
In computer programming, a callback is
executable code that is passed as an
argument to other code. It allows a
lower-level software layer to call a
subroutine (or function) defined in a
higher-level layer.
Said another way, when you pass a callback to your method, it's as if you are providing additional instructions (e.g., what you should do next). An attempt at making a simple human example follows:
Paint this wall this shade of green (where "paint" is analagous to the method called, while "wall" and "green" are similar to arguments).
When you have finished painting, call me at this number to let me know that you're done and I'll tell you what to do next.
In terms of practical applications, one place where you will sometimes see callbacks is in situations with asynchronous message passing. You might want to register a particular message as an item of interest for class B.
However, without something like a callback, there's no obvious way for class A to know that class B has received the message. With a callback, you can tell class B, here's the message that I want you to listen for and this is the method in class A that I want you to call when you receive it.
Here is a Java example of a callback from a related question.