USB-to-serial adapters for CNC: what works for DNC and drip feed

New PCs and laptops have no serial port, so most shops connect their CNC machines through a USB-to-RS-232 adapter. For sending and receiving programs that works well. Drip feed to an old control is where adapters fail: the program runs fine for an hour and then the control raises a buffer overflow near the end.

Why an adapter behaves differently from a real port

USB moves data in packets, not as a steady stream of characters. Short delays between packets do not matter when a program is stored, but a control that runs the program while it arrives can notice them.
The adapter has its own buffer. When the control sends XOFF because its buffer is full, characters already in the adapter's buffer may still go out. A control with a small buffer overflows.
Signal levels and chips differ. Cheap adapters sometimes produce weak RS-232 voltages or use counterfeit chips with unstable drivers, which shows up as random errors.
USB can disconnect. Power saving or electrical noise can make Windows drop and re-add the adapter, which stops a long transfer.

What to buy

For drip feed to old controls, a built-in serial port or a PCI or PCIe serial card is the safest choice. It behaves like the ports these controls were built for.
For a USB adapter, choose a known brand with a current Windows driver, ideally one whose driver lets you set the buffer size or latency.
For several machines, a multiport card or a network serial adapter per machine.

Users report good and bad experiences with every common adapter chipset. The deciding factor is usually not the brand but whether the driver lets you reduce the buffer and latency, and whether the adapter is genuine.

Windows settings that help

Keep the COM number fixed. Plug the adapter into the same USB socket every time; in another socket it may get another COM number, and the machine's configuration no longer matches.
Reduce the buffer and latency. In Device Manager, open the adapter's port, Port Settings, Advanced: lower the transmit and receive buffer sizes and, if offered, the latency timer. On a built-in port or card, lower the FIFO buffers the same way. One shop's old Fanuc drip-fed without errors only after the FIFO was turned down.
Stop USB power saving. In the power options, turn off USB selective suspend, and in Device Manager clear "Allow the computer to turn off this device to save power" for the adapter and the USB hub.
Use hardware handshaking (RTS/CTS) if the control and the cable support it: the adapter reacts to a line change faster than to an XOFF character.

Test before the long run

Loopback: connect pins 2 and 3 of the adapter, type in a terminal program, and every character must come back.
Send and receive a short program.
Drip-feed your longest program in a dry run.

If step 3 fails and the same transfer works from a PC with a real serial port, the adapter or its buffer is the cause. More checks for every link: RS-232 troubleshooting.

DNC Precision settings for adapters

DNC Precision works with any COM port that Windows shows, including USB-to-serial adapters. For slow or sensitive controls, set a delay before each line or between characters, use the handshake the control expects, and set waiting for XON before sending if the control needs it. The drip feed guide lists every setting.

Questions

Can I use a USB-to-RS-232 adapter to send programs to a CNC? Yes. Sending and receiving programs works with most adapters from known brands. Drip feed to old controls with small buffers is the hard case: there a built-in port or a PCI or PCIe serial card is safer, or an adapter whose driver lets you reduce its buffer and latency.

Why does drip feed overflow with a USB adapter but not with a real serial port? USB moves data in packets and the adapter keeps characters in its own buffer. When the control sends XOFF, the adapter may still send what it holds, and an old control with a small buffer overflows, often near the end of the program.

Do I need a different cable with a USB adapter? No. A USB adapter's connector is wired like a PC's serial port, so you need the same cable as before, usually a null-modem cable wired for your control.

Is a PCI serial card better than a USB adapter for CNC? For drip feed to old controls, usually yes: a card behaves like a built-in port, without USB packet delays, and its FIFO buffer can be reduced in Windows. For plain sending and receiving, a good USB adapter is enough.