The Motherboard was connected to the keyboard by a flexible circuit which was soldered in place. Aluminum overlays in the case provided ground planes which shielded the CMOS circuitry against static discharge and interference.
The motherboard had to be very small because most of the space in the bottom of the HP-71B was used by the four ROM/RAM expansion ports, the battery compartment, HP-IL expansion port and the card reader port. The motherboard contained 20 IC chips which, due to the small area, required hybrid packaging. Sets of four ICs were chip-on-board mounted to hybrid circuit boards which were in turn stacked on the motherboard.
The four bit Saturn processor was also key to keeping the motherboard small. The processor required just eleven signal lines to the surrounding ICs. The same 11 signal lines were routed to the card reader port, HP-IL port and the 4 ROM/RAM ports. (Typical 8 bit processor designs of the time would have used 30 or more lines.)
The top circuit board was dominated by the key contacts which used a snap disc design. This board also contained a number of surface mounted components including four ICs as well as resistors for static discharge protection at the key contacts.
The plug-in ROM/RAM modules used the same hybrid assemblies used for the motherboard ROM/RAM. They were encased using ultrasonic welding and used beryllium-copper spring contacts for connecting to the motherboard.
The entire card reader including magnetic head, connector, 20 discrete components and 2 hybrid ICs fit into a tiny package of under 2 cubic inches.
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