One of my biggest surprises in this project was how many design decisions there were to make, in spite of starting with a complete electrical design. Most of those decisions had to do with the physical design of the system: how to mount, package, wire, and interconnect all the components required to build the machine. Not a mechanical designer by either training or inclination, I was pleased with the solutions I worked out for the various problems encountered along the way, and although I held my breath for a long time, no serious gotchas came up that required any massive rework.
Harry built his machine in four large “shadow boxes” designed for wall mounting. He glued the ends of the relays to the plexiglass fronts of the boxes and wired directly to the relay terminals. I decided at the outset that we needed a more compact packaging that could stand alone on the museum floor, and one that would be completely maintainable over the long haul. Thus, RC-3 is housed in a standard 19″ equipment rack that is 6′ tall. Relays are socket mounted on relay rails arranged on both sides of the rack, and all wiring was done on the socket terminals, allowing relays to be unplugged and replaced if necessary.
The relay mounting rails are attached to standard rack rails that are set back 2″ from the face of the rack using studs and spacers. Plexiglass covers over the relays allow them to be seen but not touched. There are 30 relay rails, each mounting up to 16 relays. All registers, the program counter, and the incrementer, are mounted on the right side of the rack. The clock, auxiliary clock, sequencer, control, ALU, printer driver, and memory sections are mounted on the left side of the rack. The memory subsystem is mounted on a circuit board that is wire-wrapped. Connections to adjacent relay rails are made using 34-pin flat cables and Berg connectors. All indicators (except for those on the memory board and the print driver) and switches are mounted on the front panel.
Four printed circuit board designs are used for interconnections: two with different hole patterns for the rows of LEDs and switches mounted on the front panel, one for interconnections between the front panel and the rest of the machine (the front panel connector board), and one for interconnections between the front panel and other locations to the relay rails (the relay connector board). Interconnections among the various sections are made using standard 25-pin data cables with DB-25 connectors. A cable ladder running through the middle of the rack supports the weight of the cables to relieve stress on the connector boards.
The front panel is recessed 1″ into the rack, allowing it to be covered by a transparent plexiglass door when on display. Each row of LEDs and related switches (23 rows total) is mounted to a 16.85″ x 1.5″ printed circuit board on the back side of the front panel, which is supported by the switches and corner-mounted spacers. A 25-wire cable, looped in a U shape, connects the LEDs/switches to the front panel connector boards mounted on support rails 4″ behind the front panel. Each front panel connector board provides up to six DB-25 connectors for interconnection to the rest of the machine.
The three power supplies are mounted on a support plate attached to the equipment rack’s rear mounting rails. A power distribution section of the front panel mounts the AC power switch, indicator, and fuse, a relay power meter, ten DC fuses with associated indicators, and the key-switch that controls debug power. Each subsection of the machine is independently fused to allow easy isolation of any power faults. DC power is distributed from terminal strips on the power distribution panel mounted behind the power section of the front panel. Paired 12-gauge wires carry power to each relay and front panel subsection where they interconnect using mini-Molex connectors.
The relays within each functional section (up to four relay rails per section) are wired point-to-point on the relay socket terminals using standard hookup wire. Functional sections may be disconnected from the rest of the machine and removed as a unit for troubleshooting and/or repair. Each relay rail has bare power and ground buses mounted along the top and bottom of the rail, supported by six insulated standoffs. Power and ground connections are made directly to the bus wires wherever needed. Relay connector boards (up to three per relay rail) mount one or two DB-25 female connectors (as needed) to provide interconnections between the relays and the rest of the machine. The relay connector boards are attached to the relay mounting rails using two L-shaped tabs and 6-32 hardware. They have two small holes in them to pass the power/ground bus wires, and a larger hole in the middle to pass any wiring through them, as needed. Each of the 25 connector pins is brought to two solder pads for wiring to the relays.
The rack itself is mounted on casters for easy movement, as required. The casters have wheel brakes that may be used to immobilize the rack when desired. The top of the rack is covered by a perforated metal panel, and the rear door is louvered for ventilation. The bottom of the rack is open. Power dissipation averages less than 300 watts, so natural convection provides adequate cooling. When on display, the sides of the rack are covered with clear plexiglass sheets and the clear front door is closed to protect the front panel.
After the side covers were installed, we found that they damped out too much of the lovely relay-clicking sounds the machine makes while in operation, so a small microphone/amplifier was added to drive an external speaker, giving museum patrons an ear into the cabinet.
Many more pictures of the internal construction of RC-3 are included in the photo gallery.
Continued in Construction.