The RT-900 uses a baud rate of 57600, No Parity, 8 data bits, 1 stop bit. Version 4.00.22 or later
The TD-H3 uses a baud rate of 38400, No Parity, 8 data bits, 1 stop bit. Version 2.52.17 or later


To request the status of the currently active VFO, send the following two bytes to the radio:
0xAA 0x60

The radio will respond with the following 37 byte packet, the format of this packet is slightly different for the RT-900 and the TD-H3, notably, the TD-H3 endianness is BIG while the RT-900 is LITTLE.


RT-900 VFO Status Packet
------------------------
    u8 signature; // (always 0xAA)
    u8 packetType; // 0x60 (squelch closed) or 0x61 (squelch open)
    U32 rxFreq; // (32 bit little endian 10 Hz units)
    U32 txFreq; // (as above)
    U16 rxSubTone;	// little endian (<0x8000=CTCSS <0xC000=DCS >=0xC000=InvertedDCS
					// essentially bit 15 is the flag for DCS, bit 14 is the flag for code inversion)
					// the actual code is bits 13 to 0
    U16 txSubTone; // (as above)
    u8 txPower; // (0-6) 0=no transmit allowed, 1=ZERO output (no PA engaged), 2=Very Low, 3=Low, 4=Mid, 5=High, 6=Very High
    u8 unused;
    union __attribute__((packed))
    {
        u16 value; // (group nibbles: 0=No Group, 1=Group A, 2=Group B, 3=Group C, etc..) will be all 0 in VFO mode
        struct __attribute__((packed))
        {
            u16 g0 : 4;
            u16 g1 : 4;
            u16 g2 : 4;
            u16 g3 : 4;
        } single;
    } groups;
    struct __attribute__((packed))
    {
        u8 bandwidth : 1; // 0=wide 1=narrow
        u8 modulation : 2; // 0=auto, 1=fm, 2=am, 3=usb
        u8 position : 1; // 0=VFO-A, 1=VFO-B
        u8 pttID : 2; // 0 = off, 1=BoT, 2=EoT, 3=Both
        u8 reversed : 1;
        u8 busyLock : 1; // 0=off, 1=on
    } bits;
    s8 clarifier; // 100 Hz Units (signed) Frequency adjustment
    char reserved[2];
    char channelName[12]; // empty if in VFO mode (ASCII)
    u16 rssi; // signal level (0x0000 to 0x01FF) (little endian)
    u8 exNoise; // noise level (0x00 to 0x7F)

TD-H3 VFO Status Packet (Differences from the RT-900 shown as comments)
-----------------------------------------------------------------------
    u8 signature;
    u8 packetType;
    U32 rxFreq; // (32 bit big endian 10 Hz units)
    U32 txFreq; // as above
    U16 rxSubTone; // big endian (<0x8000=CTCSS <0xC000=DCS >=0xC000=InvertedDCS
    U16 txSubTone; // as above
    u8 txPower; // 0 - 255
    union
    {
        u16 value;
        struct
        {
            u16 g0 : 4;
            u16 g1 : 4;
            u16 g2 : 4;
            u16 g3 : 4;
        } single;
    } groups;
    struct
    {
        u8 bandwidth : 1;
        u8 modulation : 2;
        u8 position : 1;
        u8 pttID : 2;
        u8 reversed : 1;
        u8 busyLock : 1;
    } bits;
    char reserved[4];
    char name[12];
    u16 rssi; // signal level (0x0000 to 0x01FF) (big endian)
    u8 exNoise;