Mark and space

Mark and space are terms used in telecommunications to describe two different signal states of a communications signal, generally at the physical layer of a communications system.

The terms derive from the early days of the electric telegraph system, where the marking state would cause a mark to be output on paper, and the spacing state would create no mark.

The terms would continue to be used in systems such as RS-232, with similar conventions, that "mark" would be encoded by a negative voltage (or current flow), and "space" by a positive voltage (or no current flow).

[2] In such systems, the line is typically left in the "mark" state when idle.

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7-bit Asynchronous Data Communication surrounded by start, parity and end elements including one "0" (spacing) start element and one "1" (marking) stop element [ 1 ]
Diagram of RS-232 signalling for an uppercase ASCII "K" character (0x4b) with 1 start bit, 8 data bits, 1 stop bit. Mark and Space depict negative and positive voltage levels.