1. Separate signal transit from relative arrival
Propagation delay concerns the time associated with a signal travelling through a path. Relative arrival concerns the difference between paths being compared. Those questions can matter together, but they are not interchangeable. An isolator may need assessment for a path's transit and for alignment with another path, so the requirement must identify which relationship it actually concerns.
TI's ISO774x switching-characteristics table lists propagation delay separately from channel-to-channel skew. That separation is useful when a comparison starts from the phrase fast isolator. A delay description addresses one timing subject; a skew description addresses a defined comparison. A shorter headline cannot be treated as the answer to every relative-timing question the application might have.
Define the relevant paths before deciding which table entry applies. Two outputs on one device, outputs on separate devices and the two edge types of one signal are different comparisons. This is a logical timing exercise, not a physical terminal assignment. It establishes the subject for engineering review without deriving a bus operating rate or a permissible path difference.
2. Define the same-device channel comparison
For same-device channel skew, TI's footnote identifies outputs of one device with the driving inputs connected together. The outputs switch in the same direction and drive identical loads. Those conditions belong with the skew description. They define a controlled comparison, rather than a general statement about arbitrary signals that happen to pass through one isolator package.
The common driving-input condition matters because it makes the compared paths respond to the same input event. The identical-load condition likewise keeps the output comparison on its stated basis. If the application instead presents different events or loads, the table's description should not silently be expanded to cover that arrangement. The reviewer needs to identify how the actual comparison relates to the source conditions. A request comparing arbitrary clock and data signals may not reproduce that common-event basis. The appropriate next question is which relative-timing evidence covers those actual paths. The footnote helps identify the boundary of the published comparison; it does not automatically close the application's different comparison.
Same-direction switching must retain its timing meaning in that footnote. It should not be confused with the family's separate allocation of forward and reverse information channels. The earlier SPI-isolator guide concerns which logical signals cross the barrier in each direction. Selecting that allocation does not establish the relative edge timing described by this switching-characteristics comparison.
3. Keep part-to-part skew and pulse distortion separate
Part-to-part skew addresses a comparison across different devices. TI's footnote specifies the magnitude of the propagation-delay difference for terminals switching in the same direction under identical supply voltages, temperature, input signals and loads. Preserve all those conditions. A same-device channel-skew description should not be transferred to separate packages merely because they carry the same family name.
Pulse-width distortion is another separately named quantity. Its table definition concerns the difference between the two propagation-delay edge types, rather than a comparison between channels or devices. This explains why a pulse can need its own timing assessment even when the application's main question has been described as alignment. The subjects should remain distinguishable in the technical requirement.
Do not reconstruct channel skew by subtracting unrelated minimum and maximum propagation-delay entries. Those entries do not establish the specific matched comparison defined by the skew footnote. Likewise, pulse-width distortion does not become part-to-part skew through arithmetic on detached values. Use the source's explicit parameter and conditions for the relationship being assessed, without manufacturing another specification.
4. Hand over the actual path requirement
The reviewed ISO774x table applies to its stated supply and operating conditions. Keeping that table identity matters because a parameter name alone does not establish that another supply table or another isolator has the same basis. The sourcing question should identify the exact candidate and the applicable timing comparison, rather than copying familiar footnote wording across families.
For a serial interface, distinguish the paths that must be aligned from the complete clock-and-return-path question. A logical channel-direction match and a propagation-delay heading are useful inputs, but they do not independently establish a working protocol rate. The actual interface includes its own timing requirements and operating conditions, which this component-level distinction does not calculate.
Hand over the relevant path relationship with the source's load, transition and operating-condition basis. Engineering can then determine which exact-device evidence supports the application's requirement. This guide supplies no numerical delay, physical mapping or isolation qualification. It makes the timing comparison precise enough to assess without presenting an individual table entry as an accepted result for the assembled bus.
Sources and further reading
ISO774x SLLSEP4K physical page 19, the 5-V-supply switching table and footnotes 1–3, were reread as saved text on 15 October 2026. No timing numbers, referenced figures, physical assignments, certification scope or complete-bus timing result is inferred.