Serial ports are a common way to communicate with devices. The most common serial ports, as used by devices like Metex/Voltcraft multimeters, are RS232 interfaces.
USB to serial converters
To operate devices that have RS232 or TTL serial interfaces on computers that have only USB interfaces, converters are available. Many different chipsets exist that have slightly different properties, often supporting only a subset of what common UARTs like the 8250/16550A can do. Please note that a chipset-specific driver for the respective operating system is required (sometimes included in the OS already, e.g. mostly for Linux). Chipset and driver need to be seen as a union. The problems with some chipsets mostly seem to be more related to the quality of the driver than that of the hardware. Many original USB cables from OEM manufacturers use these converters, too, either with the chip manufacturer's or a custom USB ID.
Most of the USB to serial converter chipsets deliver 3.3V or 5V TTL level signals and an additional converter chip between signal levels usually called a Line Driver/Receiver, e.g. a Maxim MAX232, is required to interface to a RS232 or other interface that uses different signal levels.
The RS232 standard allows quite a huge span of 3 to 15V resp. -3 to -15V for the two signal levels. Some RS232 devices won't work on USB-RS232 converters because these converters tend to be at the lower end of these voltages and the devices need a higher RS232 signal voltage to work, e.g. to drive optocouplers in the interface. Sometimes a device might work on a converter of type A and won't on a converter of type B, depending on the chips used and other parameters chosen by the manufacturer.
USB to serial converter chipsets
|Manufacturer||Type||USB||Data Bits||Custom baud||Baud min.||max.||Modem||Drivers||Homepage||Remarks|
|Asix (former MOSChip)||MCS7840||2.0||5,6,7,8||yes; broken on Mac||||4 serial ports|
|MCS7820||2.0||||2 serial ports|
|MCS7810||2.0||||1 serial port|
|MCS7720||1.1||||2 serial ports|
|MCS7717||1.1||||1 serial, 1 parallel port and 3-port USB hub; out of production|
|MCS7715||1.1||||1 serial, 1 parallel port|
|MCS7710||1.1||||1 serial port; out of production
|MCS7703||1.1||||1 serial port; out of production
|Cypress||CY7C64225||2.0||8||—||300||230400||4||Win||||HW Handshake supported only up to 115200 baud.|
|CY7C65211||2.0||7,8||—?||300||3 Mbaud||4||Android, Linux, Mac, WinCE, Win XP-8||||1x I2C, 1x SPI, 10x GPIO|
|CY7C65213||2.0||7,8||||8 x GPIO|
|CY7C65215||2.0||7,8||||2 serial ports, 2x I2C, 2x SPI, 17x GPIO|
|FTDI1||many||7,8||||FTDI chips have 2 different driver models, one allowing bit-banging and other exotic features with appropriate chips.|
|FT230X||2.0||7,8||●||183.1||3 Mbaud||2||Linux, Mac, WinCE 4.2-6, WinXP-8, others|||
|FT231X||2.0||7,8||●||183.1||3 Mbaud||6||Linux, Mac, WinCE 4.2-6, WinXP-8, others|||
|FT232H||2.0||7,8||●||183.1||12 Mbaud||6||Linux, Mac, WinCE 4.2-6, WinXP-8, others||||Allows many protocols, including UART, JTAG, I2C, SPI, FT1248, bit-bang|
|FT234X||2.0||7,8||●||183.1||3 Mbaud||2||Linux, Mac, WinCE 4.2-6, WinXP-8, others|||
|FT4234H||2.0||7,8||●||183.1||12 Mbaud||6||Linux, Mac, WinCE 4.2-6, WinXP-8, others||||4x UART, 2x MPSSE (FTDI), 32x GPIO|
|Microchip||MCP2200||2.0||8; others?||●||300||1 Mbaud||2||Linux, WinXP, WinVista, Win7||||8x GPIO; custom baud rates set in non-standard way using DLL/USB commands. There exists information on the net that it is based on a PIC18F14K50 microcontroller.|
|Prolific||PL-2303x||5,6,7,8||300||115200||6||Linux, Mac, WinCE5, WinXP, WinVista-8.1||||There has been critique concerning the quality of the driver (for which OSses?).|
|PL-2303HX||1.1||5,6,7,8||● (driver!)||75||6 (12) Mbps||6||Linux, Mac, WinCE4.2-7, Win2000, XP, Vista, 8||||4x GPIO, 4x Aux GPIO; >115200 baud without control lines; Custom baud rates seem to require special driver from manufacturer.|
|PL-2303RA||1.1||5,6,7,8||● (driver!)||75||921600||6||Linux, Mac, WinCE4.2-7, Win2000, XP, Vista, 8||||With RS232 transceiver; 4x GPIO; Custom baud rates seem to require special driver from manufacturer.|
|Prolific Fake||(PL-2303)||It seems that fakes of Prolific chips with limited reliability exist.|
|SiLabs||CP21xx||2.0||device-dependant||—2||300?||up to 2 Mbps||device-dependant||Linux, Mac, Windows||||Family of several devices, 1-4 serial ports.|
|CP2102||2.0||5,6,7,8||—2||300||1 Mbps||6||Linux, Mac, Windows|||
|Sunplus||SPCP825||1.1||8||-||300||115200||-||Windows 98-XP|| ||Seems to be used by old versions of the V&A VA4000 device cable. English home page broken.|
|Texas Instruments||TUSB3410||2.0||5,6,7,8||50 (I)/1200||2 Mbps||6||Linux?, Mac?; Win||||Integrated 8052 microcontroller, I2C, IrDA up to 115200 baud, auto handshake, internal test & diagnostics, 4x GPIO; Linux and Mac drivers seem to require (free).|
|WCH IC||CH340x, CH341x||2.0||8||—||50||2 Mbps||6||Win98-Win7; (Linux)||||IrDA SIR, ...|
1 In October 2014 FTDI published Windows drivers version 2.11.0 and 188.8.131.52 that will check for fake FTDI chips. If these drivers suspect a chip to be a fake, they will overwrite the USB manufacturer and device ID of the device to 0, effectively bricking the device. Installation of these driver versions (that were distributed via Windows Update automatically for some time) is strongly discouraged! See EEVblog #676 or #FTDIgate for further information.
2 The CP2102 and CP2103 offer aliasing a standard baud rate to a non-standard baud rate by using the manufacturer's customization tool.
Modem: Not all USB to serial converters support all lines used for modem status and hardware handshake. 6 means that RTS, CTS, DTR, DSR, DCD, and RI are present. 4 means that RTS, CTS, DTR, DSR are present, 2 means RTS, CTS only.
The list above tries to include all manufacturers of such converter chips. It does not include all variants because some manufacturers have over 100!
Many USB devices that use a custom USB identity are based on these chips, too. Most manufacturers offer tools that allow setting USB properties like Vendor ID, Product ID, Product Description String, Power Descriptor, Device Release Number and Device Serial number.
Common serial to RS232 line driver/receiver chipset manufacturers
USB to HID converter chipsets
These devices are based on the USB HID device class and have no serial interface/RS232 emulation at the USB protocol level. But the chips implement access to the RS232 protocol, and communicate control and data by means of USB HID reports.
|Hoitek||HE2325U||1.1||*||||Seems to be used by older versions of the UNI-T UT-D04 device cable.|
|Unmarked||SO-20 chip||?||?||Found e.g. in the Victor 70C DMM, or the Victor 86C cable.|
|WCH||CH9325||?||*||Seems to be used by newer versions of the UNI-T UT-D04 device cable.
Appears to use a programming model that is similar with / upwards compatible with HE2325U.
|SiLabs||CP2110||2.0 compatible (implements 1.1 FS)||*||silabs.com||used in the UNI-T UT-D09 device cable, built into the UNI-T UT612 LCR meter, the Voltcraft VC-890 DMM|
The serial communication layer of libsigrok transparently supports most of the above HID chips. Those which are not supported by the serial layer can be supported by specific device drivers (Victor DMMs).