RS-232 Serial Cables and Adapters

Straight-through or null modem wiring

A DB9 serial port on a PC is wired as data terminal equipment. A straight-through male to female cable connects it to a modem or other data communications equipment. Linking two computers, or a PC to a device that is also wired as terminal equipment, needs a null-modem cable that crosses transmit and receive, and female to female or male to male cables are often built that way. Many console ports on network gear use their own cable, so read the product description for how the cable is wired.

USB-to-serial adapters: chipsets, drivers and voltage levels

Few modern computers have a COM port, so a USB-to-serial adapter creates a virtual one. The chipset inside decides which driver you need and how well the adapter behaves, and speeds such as 230 kbit/s go beyond the usual 115.2 kbit/s. Voltage is the other trap: RS-232 swings between roughly plus and minus 3 to 15 V, while microcontroller boards use 3.3 V or 5 V logic. An RS-232 adapter wired straight to such pins can damage them, so use a TTL serial adapter there.

Questions about RS-232 Serial Cables and Adapters

What is a null-modem cable?

It is a serial cable that crosses the transmit and receive lines, so two devices that both expect to be the computer can talk to each other. A straight-through cable would connect transmit to transmit and fail.

Why does my USB-to-serial adapter not show a COM port?

The driver for its chipset is probably missing. Install the driver from the adapter maker, then look for a new COM port in Device Manager. A different USB port or a restart sometimes helps.

Can an RS-232 adapter talk to my microcontroller board?

Not directly. RS-232 uses higher voltages than the 3.3 V or 5 V logic on microcontroller boards and can damage them. Use a TTL serial adapter, or a level shifter between the two.