VTI Instruments EX1629, Руководство пользователя

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324 EX1629 Onboard Memory
In this case, 2 bytes (this length does not include the length byte). The other bytes are the
CMD_RESET and CMD_GETBUF commands, in that order.
It is sometimes necessary to allocate space for a return buffer in the sent packet. All MLAN
commands will send back at least two bytes: the command performed and the response to that
command. Additional components may also be included, such as CRC bytes, data echo, or an error
message, but two bytes is the minimum.
In this instance, three bytes were returned: the length of the data, the command performed, and the
response to the command. As before, the length of the packet does not include the length byte. The
two bytes returned are “84” and “00”, where “84” is from the sent packet, the CMD_RESET
command. This is the bus master echoing the command to confirm what was sent. The “00” byte
is the response to that command, in this case ML_SUCCESS. The CMD_GETBUF is not echoed
back in the response, nor is there an error code returned, as this information would be superfluous.
This indicates that the MLAN repeater received the bus master reset and that it was successful. If
an error was encountered during this process, it might look like this:
sent packet without errors
Packet length: 3
02 84 85
got a packet without errors on receive
Packet length: 3
03 84 86 02
This time, 3 bytes were received. The “84” for CMD_RESET, and then “86 02”. From
IEEE 1451.4, Annex G, “86” is the code for CMD_ERROR. This buffer was not processed
successfully, and we have not reset the bus master. The “02” is also in Annex G, and means
“RET_BUSY, previous buffer has not been processed yet.” It will be necessary to wait until the
last command completes before processing this one. Note that this error is purely hypothetical, but
illustrates the typical format for an MLAN error.
With some basic programming completed, the more complex functions required to access a
1-Wire device can be examined.
The GET_URN function, below, is the simplest function in many ways, and the most generic.
int get_urn(int channel)
{
int sendLen, recLen = 0;
sendLen = 1; // reserve first byte for length
// clear the search state so we find the 'first' device
SendPkt[+] = CMD_RESET;
// do a reset,search and then read results
SendPkt[+] = CMD_ML_RESET;
SendPkt[+] = CMD_ML_SEARCH;
SendPkt[+] = DATA_ID;
SendPkt[+] = 0;
// request the result buffer as the last command
SendPkt[+] = CMD_GETBUF;
// set the length
SendPkt[0] = sendLen - 1;
// send and receive the frame
MLanHostPacketSend(SendPkt, channel);
recLen = MLanHostPacketReceive(RecPkt, channel, MLAN_PACKET_SIZE);
return recLen;
}
As in the pervious example, the first byte is reserved for length and increments our index as
commands are inserted into the packet.