improved version of the Lempel-Ziv algorithm) was used to compress this file. The original file size was 300 Kbytes and the compressed file size was 69 K bytes. This is a reduction in size of 69/300 = 23 of the original size. compression algorithm that is efficient and light-weight would be required for implementation in the Black Spot firmware. The LZM algorithm may not be the best algorithm for this but there are many variants that could be
The second aspect of the protocol is the logic that defines the order that packets are sent in and the states the protocol can take. This is implemented in software using a finite state machine. State machines exist in the Soft Hub and also in the camera modules. The Soft Hub state machine implements the master characteristics of the Soft Hub. It controls when camera modules can use the bus by creating time slots for the camera modules to transmit data in. The camera modules’ state machines implement the slave characteristics and prevent the camera modules from using the bus when not permitted to by the Soft Hub.
The state machine in the Soft Hub (Figure 6.9) begins in the uninitialised state. Once the communications port is chosen and opened on the PC, the Soft Hub sends a Request- Data packet (RQ) to each camera module that it believes is present. Each camera mod- ule responds with any data that it has for the Soft Hub and ends by sending a Data- TransferComplete (TXC) packet. The Soft Hub notes which of the modules responded and then sends a SetTimeSlot packet (STDM) to each camera module in the Request- DataFromOnlineModules state. This packet informs the module which time slot it must use for transmission. Finally, the Soft Hub sends a Request llData packet to the broad- cast address and this is decoded by each camera module. The first camera module begins transmission and the remaining modules transmit in their time slots.
The communications between the Black Spot camera modules and the Soft Hub has been described. The camera modules use a shared U RT bus and use a time slot mechanism to coordinate which module can use the bus at any given time. low latency bus control signal is used to allow the camera modules to signal to each other when they are using the bus. Flow control was also used to prevent data loss at the Soft Hub. The protocol is a text based protocol allowing approximately 28 centroid packets to be sent per frame. This is limited by the bandwidth of the FTDI USB bridge chip. The throughput could be increased by using a binary protocol, compression scheme, or by using a different communications