The data transfers between the controller and the expansion boards uses a serial clocked transfer method. We use a slightly modified version of the Motorola SPI (Serial Peripheral Interface) bus. To ensure maximum future versatility we have provided for a few variations on how data is transferred. Before you can start using SPxTxfrU instructions you must be sure the transfer mechanism is initialized to the correct mode. The instruction:

SPxMode m

does this. ‘m’ is the mode. For our purposes the available modes are 4, 5, 6 and 7. These correspond to 4 different bus clock (speed) rates of 1.8MHz (m=4), 920kHz (m=5), 230kHz (m=6) and 58kHz (m=7). The default mode is 6, and is the one you would normally use. Current boards are no capable of working at all in mode 4.