This article covers DB9ヌルモデムオス-メススリムラインデータ転送シリアルポートアダプター2個パック。 in detail — a small inline part that solves the classic RS-232 problem of having only a straight-through cable when the device on the other end demands crossover wiring.
Overview
The Arnorin DB9 Null Modem Adapter converts an existing straight-through serial cable into a null modem (crossover) link on the spot. One side is DB9 male, the other DB9 female, and the listed dimensions are 33 x 13 x 2 mm.
The slimline body is meant to reduce the leverage a heavy cable puts on a port. It ships as a 2-pack, the connector insulator is rated UL 94V-0, and the shell is described as fully EMI shielded.
The short verdict: this is a toolbox consumable for people who touch DB9 gear regularly. If you make one serial connection a year to a fixed device, a pre-wired null modem cable may be simpler.
If the wiring requirement changes from site to site, a straight cable plus this adapter means carrying less.
Compatibility Guide
It works with any device exposing a DB9 (D-sub 9-pin) RS-232 port. The supplied spec states the null modem configuration leaves pin 9 open.
Pin 9 carries RI (Ring Indicator) in RS-232, a signal almost nothing but a modem uses, so an open pin 9 is a non-issue for PC-to-PC links and network console ports. The flip side: if you genuinely need RI for legacy modem control, this configuration will not do it.
Usage is trivial — plug the adapter's male end onto your straight cable and its female end into the device, or the reverse. Because it is male-to-female, the overall gender of the cable run does not change.
That distinction matters: a gender changer swaps male/female, this swaps the wiring. Decide which problem you actually have; if you have both, you need a gender changer as well.
The screws are removable and reversible, so you can match either a threaded hole or a stud on the mating port. Behind a rack or on the back panel of a machine, that flexibility saves real time.
Typical targets include router and switch console ports, factory PLCs, CNC machines, older instruments (scopes, balances, environmental meters), POS terminals and UPS management ports. Modern laptops have no DB9 at all, so you would add a USB-to-serial converter first and attach this adapter to its DB9 end.
What Null Modem Wiring Actually Means
Null modem wiring crosses transmit and receive so two devices can talk directly without a modem in between: TXD and RXD are swapped, RTS/CTS are swapped when hardware flow control is used, and GND stays common. Connect two PCs with a straight cable and transmit faces transmit — nothing moves.
Insert one null modem adapter and the link comes up.
The critical caveat: two adapters in series cancel each other out. A crossover of a crossover is a straight-through cable.
The second unit in the pack is a spare, not the other end. "I used both new adapters and it still doesn't work" is the single most common failure with this class of part.
Product Info
The supplied specs are: DB9 male-to-female connector, 33 x 13 x 2 mm, quantity 2, full EMI shielding, UL 94V-0 compliance, null modem configuration with pin 9 open, and removable reversible screws. No maximum baud rate or cable-length rating is provided, so this article will not invent one.
RS-232 gets less stable as length and baud rate rise, so plan on bench-testing any long, fast run.
Pricing was ￥1,869 on Amazon JP as captured on 2026-09-06 for the 2-pack — roughly ￥935 per piece. The ASIN is B075XHWVSJ.
That is entry-level for a serial accessory, but small parts like this move with stock and sales, so check the live product page before ordering. Note that the entry labelled "Amazon US" in the source data points at the same yen figure and the same ASIN, so it is not usable as an overseas price reference.
On feedback, the listing shows 982 reviews at 4.7 out of 5 stars, 94% positive (captured 2026-09-06) — high for this price class.
Keep in mind that high-volume generic accessories often share one listing across variants, so the average may cover more than this exact item.
Alternatives Compared
There are three ways to solve the same problem: an inline null modem adapter like this one, a pre-wired null modem cable, or the USB-to-serial converter itself.
| Option | Best when | Weakness |
|---|---|---|
| Null modem adapter (this) | Reusing straight cables; wiring needs vary by site | Easy to forget or double up; one more contact point |
| Null modem cable | One fixed, repeated connection | Useless when the far end wants straight-through |
| USB-to-serial converter | Your PC has no DB9 at all | Crossing varies by model; driver dependent |
In practice, converter + straight cable + this adapter when needed gives the best coverage per item carried. Each extra connector is another contact to fail, so when a link is flaky, test without the adapter first to isolate the cause.
Best Use Cases
- Network engineers — many switch and router console ports assume crossover wiring. Two units means one in the bag, one on the desk.
- Legacy industrial maintenance — PLCs, CNC machines and old instruments vary in wiring. One straight cable plus a converter covers both cases.
- Embedded and hobby developers — having a part you can insert to test a wiring theory speeds up debugging, and the slim body keeps the bench tidy.
- Anyone who wants a spare — small parts disappear exactly when you need them; the 2-pack is cheap insurance.
Before You Buy
The most common mistake is fitting both units. Two crossovers equal a straight-through link. Use exactly one per cable run.
The second is not needing a crossover at all. Some USB-to-serial converters already cross internally, and some cables are already null modem.
Adding this on top breaks the link. Always test both with and without the adapter, and verify baud rate, data bits, parity, stop bits and flow control before blaming the wiring.
Understand the pin-9-open design. Applications that rely on RI, such as ring detection in older modem control, will not work.
Ordinary PC-to-PC and console use is unaffected, but this is not a universal RS-232 fix.
Verify before deploying with hardware flow control. The supplied data does not describe how signals beyond TXD/RXD (RTS/CTS, DTR/DSR/DCD) are handled internally, so test on the bench rather than assuming, and check the listing's wiring diagram if you need certainty.
Skip it entirely if you only use a USB-C-only laptop and never touch serial gear — without a DB9 port somewhere, this part connects nothing on its own.
Finally, manufacturer, model number and compatibility text on marketplace listings for generic accessories are sometimes wrong or recycled from another product. Confirm the item from the photos and title before ordering, and check the current displayed price, since the figure above is a point-in-time capture.
FAQ
Q. How is this different from a gender changer? A.
A gender changer only swaps male/female and leaves the wiring alone. This keeps the gender and swaps transmit/receive.
Different jobs — you may need both.
Q. Can I fit one on each end?
A. No. Two crossovers cancel out and you are back to straight-through. Use one per run.
Q. Does the open pin 9 ever matter? A.
Only where RI is actually used, mainly legacy modem control. PC-to-PC and console connections are unaffected.
Q. Is there a speed limit? A.
None is stated in the supplied data. RS-232 gets less reliable as length and baud rate increase, so validate long runs in place.
Q. How much should I trust the rating? A.
4.7/5 from 982 reviews with 94% positive as of 2026-09-06 is strong, but shared listings can pool ratings across variants — treat it as a guide.
Q. Can I use it on a PC with no DB9 port? A.
Not by itself. Add a USB-to-serial converter and attach this to its DB9 end, checking first whether that converter already crosses the lines.
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