Which APC UPS Serial Cable Do You Need?

If an APC Smart-UPS has a serial port, a standard DB9 lead may look like the obvious replacement. It is not a safe assumption. Schneider Electric warns that connecting a standard serial cable to a Smart-UPS can switch the UPS off because APC uses a proprietary serial-cable pinout. The first check is not cable length or computer port; it is the exact UPS model and the cable part number. Schneider says the supported APC serial cable has its part number marked on the connector end and advises against connecting an unmarked cable.

This guide helps you identify whether your unit needs a DB9-to-DB9 cable, a keyed RJ45-to-DB9 cable, a simple-signaling lead, a smart-signaling lead, or a USB connection. The model examples below come from Schneider's APC support table. They are not universal rules for every UPS brand or every APC generation. If the label, port, or manual disagrees with a family-level example, stop and confirm the model-specific instruction before connecting anything.

Identify the UPS port and model before buying

Record the complete UPS model from its nameplate, including letters and suffixes. A label that says only "Smart-UPS" is not enough: port families changed across product generations. Photograph the rear panel and the serial port label. Note whether the UPS port is a 9-pin D-sub connector or the keyed modular connector used by some newer Smart-UPS families. A keyed RJ45-style port may resemble ordinary network hardware, but it is not an Ethernet jack. Do not plug a network cable into it just because the plug appears to fit.

Look up the cable in the manual or official Schneider support table for that exact model. The model examples in Schneider's serial-cable alert distinguish SU, SUA, SUM, and SURT units from SMT, SMX, SMC, and SRT units: the first group is listed with DB9-to-DB9 APC serial cables; the second group uses a keyed RJ45-to-DB9 cable family. These examples show why the outside shape alone is inadequate. They do not authorise transferring one cable number to a different model family.

Write down the UPS port, the computer or server port, the required cable number, and the supported PowerChute or operating-system connection method. This separates two different questions: which connector physically fits the UPS, and which cable wiring and signalling protocol the UPS expects. Both must match.

Smart signaling and simple signaling are not interchangeable labels

Schneider's cable table distinguishes smart-signaling and simple-signaling cables. For the listed SU, SUA, SUM, and SURT DB9 families, 940-0024 or 940-1524 are identified as smart-signaling serial cables, while 940-0020 is the simple-signaling option. For the listed SMT, SMX, SMC, and SRT families, 940-0625A or 940-1525A are smart-signaling serial choices, while 940-0128D is simple-signaling. The table also lists model-dependent USB options.

"Serial" describes how data travels; it does not tell you how many UPS functions the cable and software can use. Simple signaling may support a reduced set of status and shutdown functions compared with smart signaling. The exact support depends on the UPS model, interface card, and software. Schneider's PowerChute documentation presents separate simple- and smart-signaling cable tables and warns that the cable used depends on the UPS. Follow that mapping instead of treating the words as a quality grade or assuming the more feature-rich cable works everywhere.

If your goal is graceful computer shutdown, identify the host software and the functions you need. A cable can physically connect yet fail to provide the expected status or shutdown behaviour if the signalling type, port, or software configuration is wrong. A replacement cable does not add a management card or create network monitoring; it is only the connection specified for that interface.

When the part number is 940-0024

APC 940-0024 is the smart-signaling DB9-to-DB9 cable listed for certain APC Smart-UPS units. Schneider's support pages also list it for some devices with a DB-9 serial port and in specific PowerChute configurations. That makes "940-0024" a useful search term, not a blanket compatibility promise. Some other models require a different cable, a USB cable, or the cable shipped with the UPS.

Check the full printed number on the original connector, including any suffix. Do not treat a reseller's title such as "APC serial cable" as proof of part identity. Schneider advises using a marked APC part number and lists exact numbers by product family. If the original cable is missing, verify the UPS model in Schneider's cable table or manual rather than ordering by a photo of a DB9 plug.

Also check the ends. A 940-0024 listing is commonly described as DB9 on both ends, while other APC smart-signaling cables connect a keyed UPS port to DB9 on the computer side. Your desktop may need an adapter if it lacks a native serial port, but that adapter is a separate device; it does not replace the proprietary UPS-side cable.

A USB port on the computer does not make every adapter suitable

Modern computers often lack a native RS-232 port. That does not mean you should connect an ordinary USB-to-serial converter directly to the UPS. First determine whether your UPS has an approved USB port and which cable the model requires. Schneider's table lists USB A-to-B or USB A-to-A cables for named Smart-UPS families, and it states that some Easy-UPS SRV models must use the USB or serial cable shipped with the UPS.

If the UPS supports only a proprietary serial connection, use the specified UPS cable and then choose a host-side adapter only if the manual and software support that setup. Check the adapter chipset and operating-system driver support separately. A generic USB-to-DB9 adapter may convert the computer's USB interface to RS-232, but it does not necessarily reproduce the APC pinout or signalling on the UPS side. Do not use an adapter chain as a substitute for confirming the required wiring.

If the UPS turns off or behaves strangely after connection

Disconnect the suspect cable only according to the UPS manual and site procedure; do not repeatedly reconnect it to see whether the fault happens again. Schneider's support article names an incorrect standard serial cable as a cause of a Smart-UPS shutting down. If output was interrupted, confirm the UPS and protected equipment have returned to the expected state, review the event log if available, and follow the model's restart or recovery instructions.

Avoid using a forum pinout or a hand-built DB9 lead as a quick substitute. APC's warning is specifically about the proprietary signalling connection and the possibility of the UPS switching off; matching the shell and pin count is not evidence that a cable is safe. Use the marked part number or manufacturer model table. If the correct cable cannot be identified, pause and ask the UPS manufacturer or service provider rather than probing an unknown lead.

Before trying another cable, check that the UPS model is in the correct cable table, the part number is legible, the connector type is correct, the cable is for the intended signalling mode, and the host software is configured for the actual communication port. A connection failure can also come from the wrong computer COM port, disabled service, unsupported interface, or driver issue, but rule out the dangerous cable mismatch first. If the UPS remains in an alarm state, its output is unstable, or the manual directs service, contact the manufacturer or an authorised provider rather than continuing experiments.

Keep critical loads protected while troubleshooting. Follow the site's continuity plan before changing communications wiring on a live system. Do not open the UPS cabinet or probe connector pins as a shortcut; the manufacturer's model-specific support procedure is the proper diagnostic path.

Equipvia may earn a commission from qualifying purchases through this link. The link resolved to Product ID 1600896209643 during review. CPS resolution does not verify the seller's technical claims, the cable's wiring, affiliate eligibility, or commission attribution. Check the actual selected variant and current terms before ordering.

The listing identifies the part number and shows an APC-marked 940-0024 cable, but its category attribute reads "Ammeter" and the displayed price and minimum order are far above what this cable normally costs. Treat it as a way to see the marked part, not as proof of authenticity, wiring or stock. Confirm the printed number on the connector and the signalling type against Schneider's cable table before buying.

A practical order checklist

Before placing an order, make a short compatibility record with the UPS manufacturer and full model, serial-port type, printed part number if available, signalling type, computer-side port, software, and cable length. Ask the supplier to confirm the exact APC part number and wiring family, show the marking on the cable, identify the connector gender at each end, and state which specific UPS models were tested. Request a drawing or test record if the item is described as "compatible" rather than original APC stock.

Ask whether the seller is supplying an original APC part or a third-party cable described as compatible. If the answer is "compatible," request the exact APC reference it replaces, the connector gender and type at both ends, the signalling mode, and the UPS models the supplier tested. A product title that includes "940-0024" does not establish brand authorisation, internal wiring, or tested compatibility. Keep those claims separate in your purchasing record.

For a bulk purchase, order a sample first and test it with a noncritical host and the exact UPS model under a safe, supervised procedure. Check that the UPS remains in the expected operating state, the monitoring application reads valid status, and any planned shutdown test is approved and controlled. Do not accept a supplier's claim of "universal APC cable" without the model list and exact part number. A cable that works with one DB9 UPS may still be wrong for another model with a similar connector.

Use a pre-order check instead of a connector guess

A quick way to avoid a wrong order is to write down four items before searching: the exact UPS model, the physical port on the UPS, the manufacturer-listed cable number, and the computer-side connection. Then compare the supplier page against all four. If a listing says only "DB9 serial cable," it has not yet established APC smart signaling. If it says "RJ45 cable," it has not established the keyed APC connector or the required DB9-to-RJ45 pinout. A cable is a match only when the documented part and both ends fit the actual connection path.

Treat the UPS end as the controlling compatibility point. A PC-side adapter can solve the absence of a native serial port only after the correct UPS-side cable is selected. Likewise, connector gender adapters and extension leads add more connections but do not change the wiring requirement. Do not assume that a coupler, gender changer, or ordinary extension preserves the approved cable behaviour unless the manufacturer or software documentation says it does.

Know when the cable is not the problem

If the correct cable is installed but the monitoring software cannot communicate, check the host-side setup before buying another cable. Confirm that the selected COM port is the one created by the USB adapter, that another service is not already using it, and that the application supports the UPS model and interface. Schneider's PowerChute material lists cable choices by UPS family and signalling type; a correct cable does not by itself configure software or guarantee that a particular application supports every model.

If the UPS has a management or network card, local console access through that card can have a different procedure from the direct UPS-to-computer serial connection. Follow the card's guide rather than repurposing a cable from another interface. This distinction matters in a rack where the UPS, network management card, and PDU may each expose a serial-looking connector but use different cable numbers. The port label and device manual should identify which piece of equipment the cable is for.

A short decision path

If the model is SU, SUA, SUM, or SURT, Schneider's support article points to DB9-to-DB9 APC serial part numbers and distinguishes smart from simple signaling. If it is SMT, SMX, SMC, or SRT, the same article lists keyed RJ45-to-DB9 cable numbers and separates signalling types. For other models, use the current support table or the cable supplied with the UPS.

If the UPS has a USB connection, use the model's documented USB cable. If the cable number is missing, unreadable, or inconsistent with the model, do not connect it until verified. If the cable appears to fit but the UPS shuts down, treat that as a compatibility problem and stop testing. This sequence is more reliable than choosing by connector shape, generic RS-232 labelling, or seller claims.