Question: why does the power rail and the Vcc rail not show the same voltage?
You are seeing the effect of the current consumption of your 74LS173. Using Ohms Law, the voltage drop you are seeing, which appears to be up to (5.08 V - 4.92 V =) 0.16 V, means two things:
- There must be a resistance between the measuring points.
- There must be a current flowing between the measuring points.
One point to note is that you have no appropriate decoupling capacitors close to the TTL ICs. While it's not the cause of the behaviour you observe, you may get incorrect IC behaviour as a result of their omission.
Now back to your measured voltage difference. You said:
The jumper cable is measured to be 0Ω
Although that isn't true in an absolute sense, if we assume the jumper wire itself has a truly negligible resistance, then there is non-negligible resistance elsewhere, likely in the breadboard contacts (as mentioned in the comment by vrleboss).
Since you have only measured the voltage difference, there are two "unknowns":
- the current consumption of the IC, and
- the resistance between the measuring points (not just the resistance of the jumper wire).
Having two unknowns means we cannot know which of them is unexpectedly high (or perhaps they both are) until you take more measurements.
Looking at the breadboard, there seems to be the potential (no pun intended) for a higher-than-expected current consumption due to floating inputs. The jumper wires connected to 74LS173 pin 15 and pins 9&10 (which are all inputs) seem to be unconnected and therefore floating. This situation can lead to an LS TTL IC drawing excessive current. If the IC is drawing excessive current, then a relatively small resistance could produce the voltage drop you are reporting, and the IC itself would also be noticeably warm to the touch.
Depending on your available test equipment, I suggest:
careful resistance measurement between the points where you measure the voltage drop (with power off, of course).
Try using different breadboard contacts in that IC's power path, to see if the measurements change.
careful current measurement between the points where you measure the voltage drop.
When measuring the current, you need to minimise the effects of the "burden voltage" to avoid the current measurement causing other side effects.
The TI 74173 and 74LS173 datasheet mentions a typical 74LS173 supply current of 19 mA (page 7) under specific conditions. However, if my concerns about floating inputs are correct, your circuit does not meet the conditions required to expect that current consumption, and I wouldn't be surprised if it is higher than that value on your board.