Wiring 74-Series Circuits

The year I took my digital logic class at university was the last year in which students actually built their machines physically with 74-series and wires. The curriculum has moved on to teach students to implement their machines in code with HDLs on programmable logic devices, a very practical method of constructing complex circuits. Building these circuits by hand is now somewhat rare and maybe even a lost art. Here are some of the circuits I built for the class, both on breadboards and with wire wrap.

Planning things out is the first step to speedy debugging. This circuit was built for a lab that required a lot of probing and reconfiguring of the circuit. The orange wires were ones to be reconfigured. I know you are probably cringing from the use of threaded instead of proper solid core wires. I needed a lot of wires quickly, and Ye Olde Radio Shack did not have a lot of choice.

A breadboard layout plan

Figure 1. A breadboard layout plan

A modest and mediocre build of a circuit

Figure 2. A modest and mediocre build of a circuit

Stripped insulation from wires

Figure 3. Stripped insulation from wires

The following circuit is a counter that followed a certain sequence with a binary to decimal decoder to display the states. This was one of the largest circuits built in the course. The circuit was surprisingly three dimensional. The bundle of wires was very stiff and hard to work around to make new connections or to correct connections. In the end, it was satisfying to seeing my circuit schematic in come to life as a real machine.

A large state machine

Figure 4. A large state machine

A seven-segment display to display states

Figure 5. A seven-segment display to display states

A view showing the 3D aspect of the circuit

Figure 6. A view showing the 3D aspect of the circuit

The entire counter and binary decoder machine

Figure 7. The entire counter and binary decoder machine

An isometric view of the circuit

Figure 8. An isometric view of the circuit

A DIP switch for control

Figure 9. A DIP switch for control

The display and its resistors

Figure 10. The display and its resistors

"Spine" of the machine

Figure 11. "spine" of the machine

The last circuit was a design project to create a blackjack machine. The bust limit was 16 instead of 21, since we live in base 2 fun land. Because of the extreme size and complexity of the circuit, if I were to build it on a breadboard, I decided to try wire wrapping the entire circuit on a very compact perf-board.

Wire Wrapping

Figure 12. Wire Wrapping

Wire wrapping is quite quick, compared to normal breadboarding, since you can wire more or less point to point without closing off accessibility to other circuit components. With breadboards, you need to keep your holes exposed and wires out of the way. With wire wrapping, it is hard to correct mistakes, since finding a single small wire is difficult, but you make fewer mistakes in return. The resulting circuit is very robust when done correctly. All the wires and chips are securely held in place.

The blackjack machine

Figure 13. The blackjack machine

Wire-wrap is extremely sensitive to choice of material. I learned the hard way that cheap 30 AWG wires are very hard to strip reliably without damaging the core. Ideally, wire-wrap wire should have "crisp" insulation that snaps and slides off with a normal wire stripper. The core material should not snap during stripping, wrapping, or unwrapping. Unwrapping is extremely important, since you will make mistakes.

Populated wire-wrap sockets for DIP chips

Figure 14. Populated wire-wrap sockets for DIP chips.

A big downside of wire wrap is that it is expensive. It requires the wire-wrap tool and sockets, which are not cheap and hard to find due to unpopularity. The sockets cost about a dollar each. The wire is still cheap, though.

The backend

Figure 15. The backend

A closeup of the completed wire-wrap connections

Figure 16. A closeup of the completed wire-wrap connections

Wire wrapping is an extraordinarily rewarding method of prototyping, though slow and tedious. The process feels somewhat like meditation, since a bit of patience is needed. The results are neat, compact, and impressive. The circuit I built fit in the palm of my hand, while my breadboarding colleagues completely filled their 4000 point four panel boards. For scale, the circuit was two to three times larger than the counter circuit I featured in this post. The act of wrapping wires is so simple it can be easily adapted to awkward components. If you have a large project that needs to be tested in a hurry or a one-off circuit, you can try wire wrapping instead of waiting for a printed circuit board.


Written on the 12th of October in 2014