To allow for degraded signals, the receiver accepts smaller voltages as
valid. A positive voltage of 3V or greater is a logic 0 at RX or asserted at a control
input. A negative voltage of 3V or greater (more negative) is a logic 1 at RX
or Off at a control input. The logic level of an input between -3V and +3V is
undefined.
The noise margin, or voltage margin, is the difference between the output and
input voltages. RS-232??™s large voltage swings result in a much wider noise margin
than 5V TTL or CMOS logic. For example, an RS-232 output of +5V can
attenuate or have noise spikes as large as 2V at the receiver and will still be a
valid logic 0. Many RS-232 outputs have wider voltage swings that result in
even wider noise margins. The maximum allowed voltage swing is ?±15V,
though receivers must accept voltages as high as ?±25V without damage.
Two other terms you might hear in relation to RS-232 are Mark and Space. On
the data lines, Space is logic 0 (positive voltage), and Mark is logic 1 (negative
voltage). These names have their roots in the physical marks and spaces
mechanical recorders made as they logged binary data.
Table 4-2: RS-232 uses positive and negative voltages.
%(
Logic 0 or On output +5 to +15
Logic 1 or Off output -5 to -15
Logic 0 or On input +3 to +15V
Logic 1 of Off input -3 to -15
Chapter 4
48
1 %
The TIA-232 standard includes timing specifications that RS-232-compliant
chips must meet.
Pages:
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77