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). Many original USB cables from 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 | Non-standard baud rates | Max. baud rate | Homepage | Remarks |
|---|---|---|---|---|---|---|---|
| Asix (former MOSChip) | MCS7703 | 1.1 | [1] | 1 serial port; out of production | |||
| MCS7710 | 1.1 | [2] | 1 serial port; out of production | ||||
| MCS7815 | 1.1 | [3] | 1 serial, 1 parallel port | ||||
| MCS7717 | 1.1 | [4] | 1 serial, 1 parallel port and 3-port USB hub; out of production | ||||
| MCS7720 | 1.1 | [5] | 2 serial ports | ||||
| MCS7810 | 2.0 | [6] | 1 serial port | ||||
| MCS7820 | 2.0 | [7] | 2 serial ports | ||||
| MCS7840 | 2.0 | 5-8 | yes; broken on Mac | [8] | 4 serial ports | ||
| FTDI | many | 7,8 | [9] | ||||
| Prolific | PL-2302x | 7,8 | [10] | ||||
| Prolific Fake | (PL-2302) | It seems that fakes of prolific chips with limited reliability exist.1 | |||||
| SiLabs | CP21xx | 5,6,7,8 | 2 Mbps | [11] | Family of several devices |