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Ah, it finally clicked as to what's going on there. Thanks for your help. Matt

Writing it all at once as an 8 byte chunck didn't seem to make any difference, but I did notice that if I multiply by (65536 * 65536) it works properly. After looking back at vdkeybus's code, I noticed he multiplied by that (MAXINT) rather than just 65536. I guess this basically moves the important part of the number to the upper 2 bytes of the 4 byte chunk. I don't really understand why that works, but it does, so I'm not complaining. According to the log it still reads back 00 00 for the lower 2 bytes, which it claimed that it wrote during the previous write. At any rate, it seems to work. Here's the function I ended up with (I call FSUIPC_Process after the function returns): void setAltitude(DWORD *pdwResult, double Altitude) { static struct { unsigned int lval; int hval; } val; val.hval=(int)floor(Altitude); val.lval=(unsigned int)((Altitude-val.hval)*65536*65536); FSUIPC_Write(0x0570, 8, &val, pdwResult); }

I'm trying to write a function to set altitude and I came across this thread. I've pretty much using vdkeybus's function, but I can't get it to work. After I write the two words, I read them back and print them out. It reads back the correct value for the upper word, but the fractional portion is zero. When I checked the log, it appears to be writing the correct value, but it reads back all zeros. Here's a section of the log file: 24703 WRITE0 0570, 4 bytes: 8F 02 00 00 24703 WRITE0 0574, 4 bytes: D3 00 00 00 24703 READ0 0570, 4 bytes: 00 00 00 00 24703 READ0 0574, 4 bytes: D3 00 00 00 Here's my function: void setAltitude(DWORD *pdwResult, double Altitude) { unsigned int altLowerTemp; int altUpperTemp; static struct { unsigned lval; int hval; } val; val.hval=(int)floor(Altitude); val.lval=(unsigned)((Altitude-val.hval)*65536); printf("decimal value = %f\n", Altitude-val.hval); printf("alt lower wrote = %d\n", val.lval); printf("alt upper wrote = %d\n", val.hval); FSUIPC_Write(0x0570, 4, &val.lval, pdwResult); FSUIPC_Write(0x0574, 4, &val.hval, pdwResult); FSUIPC_Process(pdwResult); FSUIPC_Read(0x0570, 4, &altLowerTemp, pdwResult); FSUIPC_Read(0x0574, 4, &altUpperTemp, pdwResult); FSUIPC_Process(pdwResult); printf("alt lower read = %d\n", altLowerTemp); printf("alt upper read = %d\n\n", altUpperTemp); } Am I missing something simple here? Thanks for your help. Matt