Mark and space

{{short description|States of a communications signal}}

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.

File:MIL-STD-188-100 char struct for async comms 1972-11-15.svg

File:Rs232 oscilloscope trace.svg "K" character (0x4b) with 1 start bit, 8 data bits, 1 stop bit. Mark and Space depict negative and positive voltage levels.]]

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).{{Cite book|last=Freeman|first=Roger L.|url=https://books.google.com/books?id=K4ribVEOhSAC&dq=mark+and+space+binary&pg=PA366|title=Telecommunication System Engineering|date=2004-06-11|publisher=John Wiley & Sons|isbn=978-0-471-45133-4|language=en}} In such systems, the line is typically left in the "mark" state when idle.{{Cite book|last1=Martin|first1=James|url=https://books.google.com/books?id=UE0gcFERpisC&dq=mark+and+space+ccitt&pg=PA54|title=Telecommunications and the Computer|last2=Martin|first2=James Thomas|last3=S.J|first3=James Martin|date=1976|publisher=Prentice Hall Professional|isbn=978-0-13-902494-8|language=en}}

"Mark" is generally identified with the binary digit "1" and "space" with the binary digit "0".