Serial voltages

This page documents the different voltage levels used for RS-232 and logic-level serial.

When talking about serial communications, the term mark and space are used to describe the two states. From the CPUs perspective, a mark is a logic 1, and a space is a logic 0.

Some manufactures use the term TTL interchangably with logic level. Since most modern devices are CMOS they actually use CMOS logic levels, which are slightly different from TTL logic levels. The Logic Threshold Voltage Levels demonstrates some of the differences.

RS-232 level serial
Wikipedia provides a decent explanation of the RS-232 voltage levels. A mark (logic 1) is represented by a voltage in the range -3v to -15v. A space (logic 0) is represented by a voltage in the range +3v to +15v. In theory, the range -3v to +3v is no-mans land, however, in practice many computers extend the range of the mark to be 0v to -15v.

Logic level serial
The raw serial signals which come from a processor chip (like the PXA-255 or the ATMega128) have a mark (logic 1) represented by Vcc (the processors supply voltage), and a space represented by 0v. For the gumstix, the PXA-255 runs at 3.3v, but the ATMega128 on the robostix runs at 5v. The serial adapters are designed to convert from logic level serial to RS-232 level serial.

Logic level RS-232
To further complicate matters, some manufacturers create something called logic-level RS-232, which is unfortunately different from Logic level serial. Logic level RS-232 has a mark being represented by 0v and a space being represented by Vcc. The Garmin GPS18 LVC is an example of a device in this category. By using an inverter, these signals become 5v logic level serial.

Gumstix Expansion Boards
The console-vx, consoleLCD-vx, console-st, console-hw and tweener expansion boards are currently the only boards which provide RS-232 level serial interfaces.

These can be plugged in directly to the serial port on the back of your PC, or into a USB-to-RS232 adapter. Both console expansion boards have a 20-pin connector on the back, which includes a 3.3v logic level version of the STUART port.

The robostix and roboaudtiostix present 5v logic level serial interfaces. In order to use these with your PC, you will need to use voltage conversion which is performed by a serial adapter.

The breakout-vx, breakout-gs, audiostix2, GPSstix, and thumbstix boards all have 3.3v logic level serial interfaces. In order to use these with your PC, you will need to use voltage conversion which is performed by a serial adapter.