Null-modem cables

Choose the physical ends before the length

The DB9 male-to-female and DB9 female-to-female listings serve different ports even when their conductor counts match. Match the pinout and ends to your equipment, then open the desired length on the family card. The resulting part page identifies the exact SKU and its inventory status.

What if short messages work but transfers stall?

An application may expect hardware flow control that a simpler crossover does not carry. Compare the equipment pin assignments, cable wiring and flow-control setting before changing the bit rate. A little traffic getting through is useful evidence, but it does not verify every control line.

Rebuilding a direct link between two older computers

For a two-computer transfer, note which program each computer uses and which side starts the session. Confirm that both machines see their own serial ports before replacing any cable. Then compare the configured port parameters and the flow-control method required by the application. If you are recreating an original setup, identify whether the crossover was in a small inline adapter attached to the old cable rather than in the cable itself. Adding that adapter to a new cable that already contains the required crossover can change the arrangement again. Keep one documented place where the crossover happens.

When two serial devices need the transmit and receive lines crossed

A DB9 host connector and DB25 peripheral connector tell you the physical ends of the path. They do not establish whether the devices need a straight cable or a null-modem crossover. The 10D1-21306 is a six-foot DB9 female to DB25 male null-modem lead. Check which handshake lines the specific application needs in addition to transmit, receive and ground. If both devices can be configured, compare their speed, parity and flow control before buying a cable of another length. Keep the confirmed pinout next to the SKU so the next replacement has the same behavior.

Which cable is a suitable spare for a frequently serviced machine?

For a frequently serviced workstation, compare more than the “null modem” wording when ordering a backup lead. Record both connector genders, the wired control signals, cable length and how the connector body fits under the service cover. If your short test lead uses an inline gender changer but the installed cable connects directly, document that difference as well. Try the spare in the actual application during an approved maintenance window and record the result. A label stating which equipment pair and transfer mode were tested gives the next technician better evidence than a spare marked simply “serial cable.”

Compare parts for your setup

Technical references

• NI guide to straight-through and null modem cables