IN145406
EIA-232-E / V.28
DRIVERS/RECEIVERS
Serial busses find their applications in longer data transmission systems compared
to parallel bus structures. The most widely used, and known, serial data transmission
standard is the RS-232 which was introduced by the Electronic Industries Association
(EIA) in 1962 for the purpose of standardizing the interface between Data Terminal
Equipment (DTE) and Data Communication Equipment (DCE).
Although originally developed for modem to terminal equipment interface is widely
used as an interface for nearly all PC peripherals (mouse, plotter, scanner, printer). Using
a common standard allows widespread compatibility plus a reliable method for
interconnecting a PC to peripheral functions.
The revised standard of 1969, EIA RS-232-C has now been superceeded by EIA-
232-D (1986) which brings it in-line with CCITT V24, V28 and ISO IS2110. The latest
revision reflects the addition of mechanical specifications for the interface connector,
loopback and test modes as the major changes. RS-232 provides a low cost
communication solution and new products are being developed with a rate faster than
over.
The RS232 standard specifies a nominal upper limit for the data signalling rate of 20
000 bits per second (20kbaud). Some applications use however data rates well beyond the
maximum 20kbaud but faster transmission standards like RS423, RS422 and RS485 must
be recommended. The cable length is in RS232C specified to maximum 15 meters but is
not mentioned in EIA-232-D, as the maximum length is normally limited by the load
capacitance from the cable (typ. 150pF/m). EIA-232-D specify a maximum generator or
driver load capacitance of 2500pF. The combination of the driver’s output current
capability, the load capacitance (cable length) and the standard’s switching requirements
should be considered in every design, especially if CCITT V28 should be met, but are
often ignored.
A driver converting a TTL/CMOS signal to RS232 levels is usually supplied from +/-
12V. A receiver converting RS232 levels to TTL/CMOS levels requires only a single +5V
supply. The receiver input impedance, specified to be in the range from 3k to 7k(ohm),
combined with the signal levels dissipate considerable power - even if no transmission
takes place.
A minimum time for the signal to stay in the transition region is specified depending
on the data rate in order to reduce susceptibility to noise during transition and maintain a
well defined signal for asynchronous applications. Also, a maximum dv/dt of 30V/µs
minimizes cable crosstalk, high frequency switching emmission and interference with other
signals.
The increasing use of laptop PCs and portable equipment with RS232 interface,
demands low power devices. Also the increasing integration of modems, PCs and their
peripherals requires elimination of passive components and single chip RS232 solutions.
IL145403 are silicon gate CMOS ICs that combine both the transmitter and receiver
to fulfill the electrical specifications of EIA Standard 232-E and CCITT V.28. The drivers
feature true TTL input compatibility, slew rate limiting outputs, 300
Ω
power-off source
impedance, and output typically switching to within 25% of the supply rails. The receivers
can handle up to
±25
V while presenting 3 to 7 kΩ impedance. Hysteresis in the receivers
aid in the reception of noisy signals. By combining both drivers and receivers in a single
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