Despite Acorn's withdrawal from the Electron peripheral market, Advanced Computer Products developed the Advanced Plus 5 (AP5) expansion, featuring Tube, 1 MHz bus and user port interfaces, which plugged into a Plus 1 cartridge socket. This provided a sufficient level of compatibility that both the 6502 and Z80 second processor products from Acorn were shown to work, providing a Tube implementation that was "as faithful as you can get", with it also being noted that the Electron being available for as little as £50 at that point in its commercial lifespan was a "very cheap way of getting a CP/M machine". Some differences in the memory map of the Electron meant that BBC Micro software would need modifications to work on the Electron with AP5. The price of the unit in late 1986 was £66.70.