Chapter 4
Because you can count on getting at most a few milliamperes from the port, use
the lowest-power components you can find and a 3V supply if possible.# !
If RS-232 doesn’t meet your circuit’s needs, an alternate interface might do the
job. For some applications, a direct connection or 5V buffers and drivers are all
that’s needed. Or a different standard interface might be more appropriate.$
If the interface is between two microcontrollers or other chips whose serial
ports use 3V or 5V logic, you may be able to connect the ports directly, output
to input, without using RS-232 at all. The outputs on some microcontrollers,
such as the 8051, are too weak to drive a cable any useful distance. In that case,
you can use a buffer/line driver such as the 74HCT541. These and similar
chips can drive cables up to 10–15 ft in many environments. At the receivers,
Schmitt-trigger inputs help to reject noise. These TTL/CMOS buffer/drivers
are cheaper than RS-232 interface chips.
. 4 # !
Table 4-4 compares RS-232 with other TIA interfaces that use unbalanced
lines. Chapter 6 has details on TIA’s balanced interfaces.
TIA-562 defines an interface for transmitting at up 64 kbps. The receiver sensi-
tivity is identical to RS-232, but the output voltages are slightly lower, with a
range from ±3.3V to ±13.2V. For data rates faster than 20 kbps, maximum
capacitance must be 1000 pF or less. Linear Technology’s LTC1385 is a
TIA-562 interface chip that operates from a single 3.3V supply.
Other alternatives use a combination of balanced and unbalanced signals. As
Figure 4-6 shows, TIA/EIA-423 (commonly called RS-423) allows up to ten
receivers and one transmitter. The drivers are unbalanced, like RS-232, but the
receivers are balanced and are identical to RS-422’s receivers, described in
Chapter 6. The receivers obtain their differential inputs using the driver’s out-
put voltage and the cable’s signal-ground wire.
TIA-530 uses balanced drivers and receivers for TX, RX, RTS, CTS, and CD,
and unbalanced lines for DTR, DSR, and RI. The result is better performance
than RS-232 at a cost of more wires in the cable. Another source for similar
standards is ITU/CCITT.