Serial port

Serial Ports are a common way to communicate with devices. The most common serial ports, as used by devices like Metex/Voltcraft multimeters, are RS-232 interfaces.

USB to Serial Converters

To operate devices that have RS-232 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 3.3 V 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 15 V resp. -3 to -15 V 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 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.

Common 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					[1]	4 serial ports
	MCS7820	2.0						[2]	2 serial ports
	MCS7810	2.0							[3]	1 serial port
	MCS7720	1.1							[4]	2 serial ports
	MCS7717	1.1							[5]	1 serial, 1 parallel port and 3-port USB hub; out of production
	MCS7715	1.1							[6]	1 serial, 1 parallel port
	MCS7710	1.1							[7]	1 serial port; out of production
	MCS7703	1.1							[8]	1 serial port; out of production
Cypress	CY7C64225	2.0	8	—	300	230400		Win	[9]	HW Handshake supported only up to 1150200 baud.
	CY7C65211	2.0	7,8	—?	300	3 Mbaud		Android, Linux, Mac, WinCE, Win XP-8	[10]	1x I2C, 1x SPI, 10x GPIO
	CY7C65213	2.0	7,8						[11]	8 x GPIO
	CY7C65215	2.0	7,8						[12]	2 serial ports, 2x I2C, 2x SPI, 17x GPIO
FTDI	many		7,8						[13]
Prolific	PL-2303x		5,6,7,8		300	115200	6	Linux, Mac, WinCE5, WinXP, WinVista-8.1	[14]	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	[15]	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	[16]	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.1
SiLabs	CP21xx		5,6,7,8			2 Mbps			[17]	Family of several devices

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.

Common Serial to RS232 Line Driver/Receiver Chipset Manufacturers

• Maxim
• ZyWyn