A backplane carries electrical signals between a CPU and external devices that need very high speed communication. The backplane must be placed immediately behind its CPU or further away (up to seven metres) in the same line. At present, the only device that can be placed on a backplane is an 8 kB RAM module. Note that a backplane can only attach to one CPU (which is selected based on the direction the backplane faces); the point of intersection behind two CPUs cannot be used as a large, fast shared memory area.