I am trying to send a mavlink command for instance
GPS_RTCM_DATA ( #233 )
flags uint8_t
len uint8_t
data uint8_t[180] RTCM message (may be fragmented)
https://mavlink.io/en/messages/common.html#GPS_RTCM_DATA
I understand uint8_ would be in a single byte unsigned int.
What does the [180] means?
The uint8_t[180] in the MAVLink GPS_RTCM_DATA message means that the data field can contain up to 180 bytes.
Beware that RTCM messages can be bigger than 180 bytes and be fragmented in
more than one GPS_RTCM_DATA message.
You can check the flags field as stated in the mavlink documentation:
LSB: 1 means message is fragmented, next 2 bits are the fragment ID,
the remaining 5 bits are used for the sequence ID. Messages are only
to be flushed to the GPS when the entire message has been
reconstructed on the autopilot. The fragment ID specifies which order
the fragments should be assembled into a buffer, while the sequence ID
is used to detect a mismatch between different buffers. The buffer is
considered fully reconstructed when either all 4 fragments are
present, or all the fragments before the first fragment with a non
full payload is received. This management is used to ensure that
normal GPS operation doesn't corrupt RTCM data, and to recover from a
unreliable transport delivery order.
I tried every but it doesn't work. Except putting it as a 180 byte arrays. The data might be only 30 bytes for example. But input with the other 150 0x00 bytes in this way, the python program accepts my command. Strangely so. I can't explain why but in this case it works.
Related
I am trying to debug a RTMP client that fails to connect to some servers. I'm using Wireshark to capture the packets and compare them with a client that connects successfully (in this case, ffmpeg).
Looking at the captured packets for a successfull connection, I noticed that, when viewing at TCP level, there is an extra byte in the payload (see pics below). The extra byte has value 0xc3 and is placed at byte 0xc3 in the payload.
I Googled the best I could to find information about extra bytes in the TCP payload, but I didn't find anything like this. I tried to look in the TCP spec but no luck either. Where can I find information about this ?
TCP-level view
RTMP-level view
This happens because the message length is larger than the maximum chunk size (as per the RTMP spec, the default maximum chunk size is 128). So if no Set Chunk Size control message was sent before connect (in your case), and the connect message is greater than 128 bytes, the client will split the message into multiple chunks.
0xC3 is the header of the next chunk, looking at the bits of 0xC3 we would have 11 000011. The highest 2 bits specify the format (fmt = 3 in this case, meaning that this next chunk is a type 3 chunk as per the spec). The remaining 6 bits specify the chunk stream ID (in this case 3). So that extra byte you're seeing is the header of a new chunk. The client/server would then have to assemble these chunks to form the complete message.
I'm attempting to get an estimated range to a BLE device, given the devices RSSI and calibrated transmission power.
The calibrated txPower is supposedly emitted as the last byte in the peripherals iBeacon advertisement packet. These packets, according to documentation i've found, should be 30 bytes in length.
However, the returned byte[] for the packet is 62 bytes in length. Because of this, the format of the advertisement data is unknown.
Why might this be happening, and is there any way to decipher the format of the 62 byte packet?
When scanning for BLE devices, Android APIs return not just the bytes for the raw advertisement PDUs but also the scan response PDUs. The latter are tacked on to the end of the former in the byte array returned by the scanning APIs.
For this reason, you cannot reliably use a negative offset from the end if the byte array to access beacon fields. Using a positive offset from the beginning is more reliable, but even this can fail if unusual PDUs are inserted before the manufacturer advertisement PDU, which is the one you care about.
For 100% reliability you must parse out all the PDUs, find the manufacturer advertisement one, and look at the bytes in that. I learned this the hard way when writing the Android Beacon Library. It is open source, so even if you want to roll your own scanning, it is a good idea to see how it does it.
i tried this example:
http://arduino.cc/en/Reference/WireRead
But if i send less than 6 bytes from the slave the master still tries to read all 6 bytes and then the read function outputs -1/255. So actually the available function is kinda useless in this case, i could rather use a for till 6.
Any idea what i am doing wrong or how i can solve this? I cannot simply just filter all 255 values because sometimes i send them. I just dont understand the library behaviour here.
Edit: The weird thing is the read function returns an int, not a byte. So i can see if its -1 or 255. And its definitly 255 instead of -1. If i try to read 7 times instead of using the available function the last reading then is -1. Does the slave send wrong bytes or do i maybe need a pullup or whats going on here?
My solution is to read until read is -1 instead of using the available function. But there must be another solution.
While this seems akward, it is the expected behaviour. The I2C protocoll does not provide any means of the slave to end the requested transmission.
The length is solely defined by the quantity parameter given to Wire.requestFrom. This way the master decides, and have to know, how much bytes the slave will send. Wire.available only signals if the previously given length is reached.
To provide variable length messages, you may chose a delimiter character, like \0 for string transmissions, or prepend the message with a byte storing the number of bytes following and stop reading if the told amount is reached.
I have a situation right now when I have after reading the byte stream from COM port in object of QByteArray type exactly and only 1 byte of data. BUT one very non-friendly protocol requires to have 9 bits of data after reading data from COM port.
But according to win32API function: ReadFile(....) I can read from the COM stream ONLY bytes= 1,2,3.....
So - That's why I am reading only 8 bits=1 byte with help of this function and with help of some operations with parity bit I am calculating the value of the 9th bit of generalized data...
So on one hand I have 1 byte (8 bits) of proper(real) data - on another hand I have a value of this 9th bit (0 or 1); 2 objects which in sum must create the value of generalized data.
How I can combine these objects into one & final QByteArray object? Because the global function ReadComData can and must return only QByteArray object.
UARTs cannot "write" 9-bit data. On the wire, your (typically 8-bit) data are usually framed between a start-bit and a stop-bit, so you have 10 bits transmitted for every byte you send. If you have a parity bit, it is transmitted after the last data bit, but before the stop bit. But this is generated by the sending UART, not part of a protocol. A data bus for a typical UART 16550 is only 8-bits wide (you can actually send 5-, 6-, 7-, or 8-bit data).
On the receiving end, the UART has to be configured based on what is on the wire. If your sender is using a parity bit, then you program the UART (via the "COM" port settings) accordingly. The parity bit is just to help check for errors on the wire. It is based on the data bits -- you cannot put another data bit in a parity bit. The receiving UART can be used to check for parity errors (read via the line status register (LSR)), and this can be passed up to you via system calls.
It is possible your protocol is splitting up the data across multiple bytes. If that's the case, then convert two bytes into one 16-bit word and mask the 6 bits you don't want to use.
Is it within TCP standard that multiple messages, sent from server to client in a row, will be accepted by client at same order (and bytes of one message will not be scattered within other messages)?
TCP provides an in-order byte stream delivery service. The bytes won't arrive in another order but the number of writes need not be equal to the number of reads.
You will never read bytes in another order than that in which they were sent
You can make no assumptions on "messages". TCP doesn't know about messages, only bytes (see above). Both the sender and the receiver can coalesce and split such "messages"
TCP uses a sequence number to identify each byte of data. The sequence number identifies the order of the bytes sent from each computer so that the data can be reconstructed in order, regardless of any fragmentation, disordering, or packet loss that may occur during transmission.
I agree with #cnicutar.
How are you deserializing the objects? I suspect the problem lies there.
For example if your messages are like
ABCD followed 200 ms later by PQR. It may appear as:
ABC followed by PQR
or ABCDPQR
or even AB followed by CD followed by PQ followed by R.
Basically you cannot make assumptions based on time of receiving the data.
The deserialization logic should know the object boundaries within a stream of bytes. This information should be encoded into the stream by the serialization logic.
If you are using Java, you can use ObjectInputStream & ObjectOutputStream and not be bothered about serialzation issues.
J2ME Polish has a good serialization utility that can be very easily ported to other platforms. I have myself used it in live environment.