What is DSM (DSM2/DSMX)
DSM2 and DSMX are widely used radio protocols for 2.4gHz spektrum RC transmitters and receivers. It is also used in the SuperbitRF project as datalink. The protocol makes it possible to have multiple receiver/transmitters in the 2.4gHz spektrum without much interference.
What is the difference between DSM2 and DSMX
There is only a small difference between DSM2 and DSMX, which is the way they hop between different channels. In the DSM2 protocol the transmitter will choose two random channels, where the transmitter will look for the two best channels in the optimal case. In the DSMX protocol the transmitter and receiver both use the transmitter radio chip ID,which is send during the binding process, for generating 23 channels. Each time the transmitter transmits a packet or the receiver receives a packet they will hop to the next channel.
From this information you can conclude that when you use DSM2, and both the channels that were chosen are getting used by alot of other users your reception will drop enormously. In the DSMX protcol this problem was solved and makes it possible to have alot more transmitters/receivers at the same bandwith.
How does it work
Note that this is reverse engineered and will contain errors.
In general you have three fases, the binding procedure, the syncing procedure and the normal transfer procedure. The transmitter doesn't have a syncing procedure, because it determines the timing for the receiver.
The binding procedure
At the binding procedure both the transmitter and receiver will change to SDR mode, a data code of 64 bits, a SOP Code of 64 bits and disable the CRC Checksum. The transmitter will then choose a random odd channel and starts sending the binding packet at a very high rate. The receiver will start at the first odd channel and waits a bit longer then the rate the transmitters sends at. When it does receive a packet, it will check if it is a valid bind packet and goes to syncing mode. When it doesn't receive a valid packet it will go to the next channel and will repeat this.
During the binding procedure the transmitter also sends the amount of RC channels that it has and in which amount of packets this channels will fit. They will either transmit all the channels in one packet or in two packets after eachother.
The syncing procedure
Because the transmitter doesn't have a syncing procedure, the transmitter is already in the transfer procedure. The receiver on the other hand needs to synchronize itself with the transmitter.
The transfer procedure
Some general information about the DSM2/DSMX protocol.
- Channel: The channel in the 2.4GHz band(2.400GHz to 2.483GHz) which is used to transmit or receive. Each channel is 100mHz higher then the previous one starting by 0, and the 2.4gHz band is devided into 98 channels.
- (Transmission) Mode: The mode at which the transmitter, or receiver is sending/receiving. There are four differnt modes the transmitter/receiver could be in: GFSK, 8DR, DDR, SDR. The ones used are 8DR and SDR, in SDR mode one bit is encoded in each derived code symbol transmitted. In 8DR mode eight bits are encoded in each derived code symbol transmitted.
- SOP code: Start Of Packet code, are one or two bytes which are added at the start of the packet by the radio chip. These bytes are used to identify if a packet is received at a certain channel.
- Packet length: The packet length(one byte) is send after the SOP code, and only includes the length of the payload data. Because it is one byte the limit of the payload length is 255, but because 8DR and SDR only transmit 16byte packets they will be automaticly split up into multiple packets.
- CRC (Checksum): The CRC Checksum are two bytes which are send at the end of the packet to check if the received packet was valid. It can detect the following errors: any one bit error, aby two bits error, any odd number of bits in error and an error burst as wide as the checksum itself.
Here is an image of an example packet including the Preamble, SOP, Length, Payload Data and the CRC.
The binding packet has a length of 16 bytes, the exact size the SDR can send at once.
 radio chip ID byte 0 (inverted)  radio chip ID byte 1 (inverted)  radio chip ID byte 2 (inverted)  radio chip ID byte 3 (inverted)  radio chip ID byte 0 (inverted)  radio chip ID byte 1 (inverted)  radio chip ID byte 2 (inverted)  radio chip ID byte 3 (inverted)  (sum 00->07 + 384 - 16) MSB  (sum 00->07 + 384 - 16) LSB  0x01 (not known yet)  number of RC channels  protcol used (0x01:DSM2 1packet, 0x02: DSM2 2packets, 0x10: DSM2 SuperbitRF, 0x11: DSMX SuperbitRF, 0xA2: DSMX 1packet, 0xB2: DSMX 2packets)  0x00 (not known yet)  (sum 00->13 + 384 - 16) MSB  (sum 00->13 + 384 - 16) LSB