Two D-Sub cables can use the same connectors and still be electrically different. Straight-through, crossover, and null-modem describe how the cable routes signals. For serial equipment, choose the wiring from device roles and the approved signal map, not from connector appearance.
Straight-through: preserve the intended signal path
A straight-through cable generally carries corresponding signals without the deliberate transmit/receive crossover used to connect like serial device roles. In a same-connector assembly, this may look like pin-to-same-pin wiring, but do not assume that rule for every transition cable.
For mixed connector formats, a functionally straight-through serial cable can map a signal to a different physical pin number at the other end. Signal names matter more than equal pin numbers.
Crossover: selected signals change sides
A crossover cable intentionally exchanges selected signal paths. Which lines cross depends on the equipment and interface, so the word ‘crossover’ is not a complete manufacturing instruction.
Ask for the actual wiring table. This prevents a supplier from applying a familiar crossover pattern that does not match the customer’s equipment.
Null modem: a serial use case, not one universal drawing
Null-modem wiring commonly connects serial devices with like roles, but implementations can differ in how they handle data and handshake signals. Some applications use minimal data lines; others require additional handshaking.
The safe approach is to verify the required handshake behavior and reproduce it on the cable drawing.
DTE and DCE explain why the wiring changes
In traditional RS232 terminology, DTE and DCE roles help explain which side transmits and receives on particular signal names. Understanding those roles is more useful than memorizing one cable diagram.
For modern or proprietary equipment, the manual may not use classic DTE/DCE terminology. In that case, follow the documented signal names and pin assignments.
How to verify an unknown cable
If an existing cable works, continuity-map it end to end and record any jumpers. Do not assume the replacement is straight-through because the connector genders look conventional.
For production, convert the measured mapping into a controlled drawing and verify a first article before releasing volume.
Choose by the interface you actually have
Assuming male-to-female means straight-through; specifying ‘null modem’ without a wiring table; and ignoring handshake requirements.
| Decision input | What you need to confirm |
| Check 1 | Device A role/signals |
| Check 2 | Device B role/signals |
| Check 3 | Straight/crossed lines |
| Check 4 | Handshake requirement |
| Check 5 | Approved wiring map |
Before you release the order
Use the DB9 RS232 pinout to confirm the first related decision, then review the D-Sub pinout guide before you freeze the order.
How BespokeCable can review the wiring requirement
For a device-specific cable, you can send BespokeCable the equipment models, the pin-to-pin map, and a verified sample when documentation is incomplete. The published customization process moves from inquiry and specification review to quotation, sample approval, and then production, which gives you a clear point to resolve wiring questions before a batch is released.
Get the fan-out or transition reviewed
Send BespokeCable your equipment models, connector details for both ends, approved pinout or verified sample, finished dimensions, shielding and mechanical requirements, quantity, and sample-approval criteria. You can then review a defined assembly instead of relying on a generic D-Sub description.
