May 2014
Intermediate to advanced
320 pages
13h 41m
English
Content preview from Servicing Personal Computers, 3rd Edition
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58 Servicing Personal Computers
Figure 1.64. RS-232 ports (COMI and COM2) in an IBM PC/XT compatible (note the line drivers and
line receivers, U3 to U6)
The voltage levels in an RS-232C system are markedly different from those within the
computer. In the transmit and receive data paths, for example, a positive voltage of between
3 V and 25 V is used to represent logic 0 while a negative voltage of similar magnitude is used
to represent logic 1. In the control signal paths, however, conventional positive logic is
employed;
a high voltage in the range 3 V to 25 V indicates the active or 'on' state while a
negative voltage of similar magnitud ...