A DB9 connector is commonly used for RS232, but the connector doesn’t confirm the port is RS232, and it doesn’t indicate whether the cable should be straight-through or null modem. For replacement and OEM work, verify the device role, active signals, and handshake requirement before freezing the pinout.
DB9 is the connector; RS232 is the interface
Keep the mechanical and electrical definitions separate. A 9-position D-Sub can be wired for RS232 or for a proprietary interface. Confirm the equipment documentation before applying a standard serial pinout.
This distinction matters in industrial equipment, where a familiar connector may be used for service, programming, or machine-specific I/O.
Common RS232 signal names
RS232 documentation commonly uses signal names such as TXD, RXD, RTS, CTS, DTR, DSR, DCD/Carrier Detect, RI, and signal ground. Not every application uses every handshake line.
For a custom cable, list only the signals actually required by the equipment and show their physical contact numbers on each connector.
DTE and DCE affect cable selection
Traditional RS232 device roles help determine which signals must connect directly and which may need to cross over. A cable that works between DTE and DCE equipment may not work between two devices with the same role.
When the manual identifies DTE/DCE, include that information in the engineering review. When it does not, rely on the documented TX/RX and handshake assignments.
Straight-through vs null modem
A straight-through serial connection preserves the intended complementary device roles. A null-modem connection connects like roles by crossing selected signals. The exact null-modem implementation can vary, especially for handshake lines.
Do not specify only ‘null modem’ for a production cable. Provide the required pin-to-pin map.
Handshake lines are a common replacement trap
Some equipment communicates only with TX, RX, and ground. Other equipment expects RTS/CTS or DTR/DSR-related behavior. A cable that carries the data pair correctly can still fail if the device expects hardware handshaking.
Check the manual or known-good cable before simplifying the wiring.
What to send for a custom DB9 RS232 cable
Provide both connector genders, device models, DTE/DCE information if known, required pinout, cable length, shielding, backshell or panel requirements, and quantity. If replacing an old cable, add continuity results or the original cable for verification.
Approve a sample on the real equipment before volume production when the interface is legacy or poorly documented.
Questions buyers ask when the name is not enough
Does every DB9 serial cable use all nine pins?
No. The active signals depend on the equipment and handshake method.
Is a null-modem cable the same as a crossover cable?
A null modem is a serial crossover use case, but the exact handshake wiring can vary.
Can I use a generic DB9 cable for PLC programming?
Only if the PLC documentation confirms the required connector, interface, and pinout.
Continue the specification
Use the straight-through vs crossover vs null modem to confirm the first related decision, then review the custom D-Sub cable quote 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.
Send the interface evidence, not only the connector name
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.
