USB to Serial Cable: One Piece or Two?
Two products do the same job: an adapter with a DB9 socket and a separate cable, or a single lead with the chip built into the plug. The difference shows up in the field.
Quick Answer
A USB to serial cable has the bridge chip moulded into the housing and terminates directly in a DB9 connector, so there is one less joint to fail. An adapter plus a separate lead is more flexible because the lead can be swapped for a null modem or a longer version. Both create a virtual COM port and both run at RS-232 speeds, which are far below what USB provides.
Comparing the Two Approaches
| Attribute | One piece cable | Adapter plus cable |
|---|---|---|
| Connections | Two, host and equipment | Four, including the DB9 coupling |
| Reliability | Higher, fewer joints | Lower, more points of failure |
| Flexibility | Fixed length and gender | Swap cables for null modem or length |
| Field replacement | Replace the whole unit | Replace one part |
| Cost | Similar | Similar |
Wiring Options to Know
Straight through
Pin to pin, used when connecting a computer to a modem or to equipment that acts as a DCE device.
Null modem
Receive and transmit crossed, with handshake lines looped. Needed for computer to computer or controller to controller links.
Gender
Most adapters present a male DB9, so equipment with a female socket is the normal pairing.
Handshake lines
Only pins 2, 3 and 5 carry data and ground. The rest are status and flow control, which some equipment requires.
Rate Expectations
| Layer | Rate | Practical effect |
|---|---|---|
| USB 2.0 bus | 480 Mbps | Never the limit |
| Typical serial setting | 9600 bps | Console and instrument work |
| High serial setting | 115200 bps and above | Firmware uploads |
| RS-232 cable limit | About 15 m at moderate rates | Longer runs need converters |
Real-World Applications
- A single lead in a technician toolkit for console access.
- Fixed installation wiring for a PLC programming point.
- Connecting an instrument with a female DB9 socket.
- Long term bench setups where fewer joints matter.
Related Guides
For adapter selection, read USB serial adapter. For the USB-C host version, see USB-C to serial adapter. For connector detail, start with USB to DB9 serial adapter.
FAQ
Is a USB to serial cable the same as an adapter?
Functionally yes. The cable simply has the chip built into the housing rather than a socket to plug into.
Why does the equipment not respond?
Usually the wrong baud rate, a missing handshake line, or a straight cable where a null modem is required.
Can I extend the DB9 end?
Yes, up to the RS-232 length limits, but each extension adds a joint and potential noise pickup.
Do I need a specific driver for a one piece cable?
Yes. The driver depends on the chipset inside the moulding, not on the cable type.