Wire Wrap - Design Automation

Design Automation

In wire-wrapping, electronic design automation can design the board, and optimize the order in which wires are placed.

The first stage was that a schematic was encoded into a netlist. This step is now done automatically by EDA programs that perform "schematic capture". A netlist is conceptually a list of pins, with each pin having an associated signal name.

The next step was to encode the pin positions of each device. The easy way to do this is to encode lettered rows and numbered columns where the devices should go. The computer then assigns pin 1 of each device in the bill of materials to an intersection, and renames the devices in the bill of materials by their row and column.

The computer would then "explode" the device list into a complete pin list for the board by using templates for each type of device. A template is map of a device's pins. It can be encoded once, and then shared by all devices of that type.

Some systems optimized the design by experimentally swapping the positions of parts and logic gates to reduce the wire length. After each movement, the associated pins in the netlist would be renamed. Some systems could also automatically discover power pins in the devices, and generate wires to the nearest power pins.

The computer program then merges the netlist (sorted by pin name) with the pin list (sorted by pin name), transferring the physical coordinates of the pin list to the netlist. The netlist is then resorted, by net name.

The programs then try to reorder each net in the signal-pin list to "route" each signal in the shortest way. The routing problem is equivalent to the travelling salesman problem, which is NP complete, and therefore not amenable to a perfect solution on a reasonable time scale. One practical routing algorithm is to pick the pin farthest from the center of the board, then use a greedy algorithm to select the next-nearest pin with the same signal name.

Once routed, each pair of nodes in a net becomes a wire, in a "wire list". The computer then reads incidental information (wire color, order in the net, length of the wire, etc.) in the netlist and interprets it to renumber the wire list to optimize the ordering and direction of wires during production. The wire list is then resorted by the wire numbers.

For example, wires are always "top and bottomed". That is, wires alternate between high and low as they connect a series of pins. This lets a repair or modification occur with the removal of at most three wires.

Long wires are usually placed first within a level, so that shorter wires will hold longer wires down. This reduces vibration of the longer wires, making the board more rugged in a vibrating environment such as a vehicle.

Placing all the wires of a certain size makes it easier for a manual or semiautomated wire-wrapping machine to use precut wire. This especially speeds up manual wrapping.

Wires of different colors can also be placed together. Most wires are blue. Power and ground wires are often made with red and black. Clock wires (or other wires needing special routing) are often made yellow or white. Twisted pairs are usually black and white.

Another optimization is that within each size and color of wire, the computer selects the next wire so that the wrap head moves to the nearest pin. This can save up to 40% of the wrap time, almost getting two wire-wrap machines for the price of one. It also reduces wear on the wire-wrap machines.

Finally, the direction of placing a wire can be optimized for right-handed wire-wrap people, so that wires are placed from right to left. In a semi-automated wire-wrap system, this means that the wrap head moves away from the user's hand when placing a wire. The user can then use their strong hand and eye to route the wire.

Lastly, the sorted, optimized wire list is then printed out for use by machine operators, and turned into a tape or card deck for the machine. Machine-readable copies of this valuable production data are often archived at the same time.