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參數(shù)資料
| 型號： | DS276 |
| 廠商： | DALLAS SEMICONDUCTOR |
| 元件分類： | 通用總線功能 |
| 英文描述： | Low Power Transceiver Chip(低功率的RS232收發(fā)器芯片) |
| 中文描述： | LINE TRANSCEIVER, PDIP8 |
| 封裝： | 0.300 INCH, DIP-8 |
| 文件頁數(shù)： | 2/11頁 |
| 文件大?。?/td> | 89K |
| 代理商： | DS276 |
DS276
021998 2/11
OPERATION
Designed for the unique requirements of battery–
backed systems, the DS276 provides a low–power full–
or half–duplex interface to an RS–232 serial port. Typi-
cally, a designer must use an RS–232 device which
uses system power during both negative and positive
transitions of the transmit signal to the RS–232 port. If
the connector to the RS–232 port is left connected for an
appreciable time after the communication session has
ended, power will statically flow into that port, draining
the battery capacity. The DS276 eliminates this static
current drain by stealing current from the receive line
(RX
IN
) of the RS–232 port when that line is at a negative
level (marking). Since most asynchronous communica-
tion over an RS–232 connection typically remains in a
marking state when data is not being sent, the DS276
will not consume system power in this condition. Sys-
tem power would only be used when positive–going
transitions are needed on the transmit RS–232 output
(TX
OUT
) when data is sent. However, since synchro-
nous communication sessions typically exhibit a very
low duty–cycle, overall system power consumption
remains low.
RECEIVER SECTION
The RX
IN
pin is the receive input for an RS–232 signal
whose levels can range from
±
3 to
±
15 volts. A negative
data signal is called a mark while a positive data signal is
called a space. These signals are inverted and then lev-
el–shifted to normal +3 or +5 volt CMOS/TTL logic lev-
els. The logic output associated with RX
IN
is RX
OUT
which swings from V
CC
to ground. Therefore, a mark on
RX
IN
produces a logic 1 at RX
OUT
; a space produces a
logic 0.
The input threshold of RX
IN
is typically around 1.8 volts
with 500 millivolts of hysteresis to improve noise rejec-
tion. Therefore, an input positive–going signal must
exceed 1.8 volts to cause RX
OUT
to switch states. A
negative–going signal must now be lower than 1.3 volts
(typically) to cause RX
OUT
to switch again. An open on
RX
IN
is interpreted as a mark, producing a logic 1 at
RX
OUT
.
TRANSMITTER SECTION
TX
IN
is the CMOS/TTL–compatible input for data from
the user system. A logic “1” at TX
IN
produces a mark
(negative data signal) at TX
OUT
while a logic 0 produces
a space (positive data signal). As mentioned earlier, the
transmitter section employs a unique driver design that
can use the RX
IN
line for swinging to negative levels.
RX
IN
can be connected via external circuitry to V
DRV–
to
allow stored charge to supply this voltage during mark-
ing (or idle) states. When TX
OUT
needs to transition to a
positive level, it uses the V
DRV+
power pin for this level.
V
DRV+
can be a voltage supply between 3 to 12 volts,
and in many situations it can be tied directly to the V
CC
supply. It is important to note that V
DRV+
must be greater
than or equal to V
CC
at all times.
The voltage range on V
DRV+
permits the use of a 9V bat-
tery in order to provide a higher voltage level when
TX
OUT
is in a space state. When V
CC
is shut off to the
DS276 and V
DRV+
is still powered (as might happen in a
battery–backed condition) , only a small leakage current
(about 50–100 nA) will be drawn. If TX
OUT
is loaded dur-
ing such a condition, V
DRV+
will draw current only if RX
IN
is not in a negative state. During normal operation (V
CC
= 3 or 5 volts), V
DRV+
will draw less than 2 uA when
TX
OUT
is marking. Of course, when TX
OUT
is spacing,
V
DRV+
will draw substantially more current – about 3 mA
depending upon its voltage and the impedance that
TX
OUT
sees. The TX
OUT
output is slew–rate limited to
less than 30 volts/us in accordance with RS–232 speci-
fications. In the event TX
OUT
should be inadvertently
shorted to ground, internal current–limiting circuitry pre-
vents damage, even if continuously shorted.
RS–232 COMPATIBILITY
The intent of the DS276 is not so much to meet all the
requirements of the RS–232 specification as to offer a
low–power solution that will work with most RS–232
ports with a connector length of less than 10 feet. As a
prime example, the DS276 will not meet the RS–232
requirement that the signal levels be at least
5 volts
minimum when terminated by a 3K
load and V
DRV+
=
+3–5 volts. Typically 2.5 to 4 volts will be present at
TX
OUT
when spacing under this condition, depending
on the supply voltage. However, since most RS–232
receivers will correctly interpret any voltage over 2 volts
as a space, there will be no problem transmitting data.
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