FAQs on USB (Universal Serial Bus)

Q: My computer has only a 9 pin serial port but my new device (Printer, Camera, etc.) has only USB Connections. Do you have a converter that can be used to connect them?

A: You need a USB Host Connection to support the USB device. A USB Host Connection cannot be supported by RS-232 Serial port connections since the USB host port requires a microprocessor and software to drive it. In theory, a USB Serial Server is possible, but not practical since special drivers would still be needed on the computer for each device.

Q: My new laptop computer has only a USB port, but I need to connect a 9 pin serial device. How can I connect to it? What can I use?

A: Use a USB to RS-232 Converter. (such as USO9ML2 or 232USB9M) This will work well with recent software written for Windows. Older DOS based software which doesn't use the Windows drivers may need to use a PCMCIA card. (SSP-100, DSP-100).

Q: What USB to serial converters are recommended for use with Windows XP?

A: The USO9ML2 USB to RS-232 and USOPTL4 & USPTL4 USB to RS-422/485 Converters all have XP compatible drivers.

Q: Does a USB to RS-232 Converter work both ways?

A: The RS-232 connections provide standard inputs and outputs that match a regular 9 pin or 25 pin serial port. A USB converter must connect to a USB Host port. The USB connector (Type A Male) only fits upstream device connectors (Type A Female) such as a Host port. USB Device (slave) devices which don't have a cable attached have a downstream (Type B Female) connector.

A: What are USB Type A or B Male and Female Connectors?

Q: Type A are Upstream connectors toward Host computer. Type B are Downstream connectors toward Devices. The USB connectors & cables carry power, ground and a differential pair of data lines. USB connectors are designed so you can only make proper connections between the USB Host and USB (slave) Devices. Two USB devices can only communicate with the Host, not directly with each other.

USB-A Female Connector USB-A Male Connector
USB-B Female Connector USB-B Male Connector

Q: Can I use USB devices with NT 4.0?

A: Microsoft does not support USB in NT 4.0 

Q: How do I access a USB Serial Port Converter?

A: The same as any other Windows based Com port. Software that runs in Windows using standard Windows drivers will be able to access the port as another Com number. Windows supports Com1 to Com255, usually a USB device will become the next available com port. This can be Com3, Com4, Com5, Com6 or higher. Properly designed Windows based applications will be able to access any port number unless limited to Com1 to Com16 or less.

Q: How does your USB to RS-422/RS-485 Converter show up on my system?

A: After the drivers are installed, it looks like another Com port with a different number. If you install more units by connecting them, each unit will be assigned the next available Com number. The new com port shows up in the Control Panel, System, Device Manager, Ports as a Com number. The USOPTL4 and USPTL4 converters support Receive and Transmit signals using RS-422 operation or 2-wire or 4-wire RS-485 mode using Automatic Send Data Transmitter/Receiver Control.

Q: How compatible are USB based Serial Ports? What should I be aware of?

A: Applications written for Windows 98/SE/ME/XP or 2000 which use the standard Windows drivers will work properly. Older software written for DOS or which only works with Com1 to Com4 in Windows 95/98/SE/Me may have problems. This includes many PLC utilities and software attempting RS-485 transmitter control using RTS. Automatic Send Data Control is recommended for RS-485 use in Windows. Contact the software manufacturer to inquire if their application works with USB & PCI based Com ports. If not, you will need to use a motherboard or ISA bus com port set to a legacy address and IRQ.

Q: How many USB based Serial Ports can I add?

A: If you have enough computer resources, one USB port can support up to 127 devices using USB Hubs to obtain additional connections. 

Q: Where can I get more information about USB?

A: At the USB website: