MAX-232:
Serial RS-232 (V.24) communication works with
voltages (-15V ... -3V for high [sic]) and +3V ... +15V for low [sic]) which
are not compatible with normal computer logic voltages. On the other hand,
classic TTL computer logic operates between 0V ... +5V (roughly 0V ... +0.8V
for low, +2V ... +5V for high). Modern low-power logic operates in the range of
0V ... +3.3V or even lower.
So, the maximum RS-232 signal levels are far too high for
computer logic electronics, and the negative RS-232 voltage for high. In the
other direction (sending data from some logic over RS-232) the low logic
voltage has to be "bumped up", and a negative voltage has to be
generated, too.
Logic Voltages
All this can be done with conventional analog electronics,
e.g. a particular power supply and a couple of transistors or the once popular
1488 (transmitter) and 1489 (receiver) ICs. However, since more than a decade
it has become standard in amateur electronics to do the necessary signal level
conversion with an integrated circuit (IC) from the MAX232 family (typically a
MAX232A or some clone). In fact, it is hard to find some
The MAX232 &
MAX232A
The MAX 232 translates RS232 voltages to TTL voltages.
RS232 represent a binary 1 or HI anywhere between –3V to –12V, a zero logic or
LOW, between 3V and 12V.
TTL in the other hand responds to 0 to 2.1V as logic zero
and 2.8V to 5V as a HI. The MAX 232 provides voltage translation so the TTL PIC
16F84 can understand the messages sent to it from the computer. A serial cable
is also provided to connect the MAX232 to the PC and jumper cables to connect
the MAX232 to the micro controller.
The MAX232 from Maxim was the first IC which in one package
contains the necessary drivers (two) and receivers (also two), to adapt the
RS-232 signal voltage levels to TTL logic. It became popular, because it just
needs one voltage (+5V) and generates the necessary RS-232 voltage levels
(approx. -10V and +10V) internally. This greatly simplified the design of
circuitry. Circuitry designers no longer need to design and build a power
supply with three voltages (e.g. -12V, +5V, and +12V), but could just provide
one +5V power supply, e.g. with the help of a simple 78x05 voltage converter.
MAX232
(A) DIP Package
DIP Package of MAX 232A
A Typical Application
The MAX232 (A) has two receivers (converts from
RS-232 to TTL voltage levels) and two drivers (converts from TTL logic to
RS-232 voltage levels). This means only two of the RS-232 signals can be
converted in each direction.
There are not enough drivers/receivers in the MAX232 to
also connect the DTR, DSR, and DCD signals. Usually these signals can be
omitted when e.g. communicating with a PC's serial interface. If the DTE really
requires these signals either a second MAX232 is needed, or some other IC from
the MAX232 family can be used (if it can be found in consumer electronic shops
at all). An alternative for DTR/DSR is also given below.
Maxim's data sheet explains the MAX232 family in great
detail, including the pin configuration and how to connect such an IC to
external circuitry. This information can be used as-is in own design to get a
working RS-232 interface. Maxim's data just misses one critical piece of
information: How exactly to connect the RS-232 signals to the IC. So here is
one possible example:
|
MAX232 to RS232 DB9
Connection as a DCE
|
||||
|
MAX232 Pin No.
|
MAX232 Pin Name
|
Signal
|
Voltage
|
DB9 Pin
|
|
7
|
T2out
|
CTS
|
RS-232
|
7
|
|
8
|
R2in
|
RTS
|
RS-232
|
8
|
|
9
|
R2out
|
RTS
|
TTL
|
n/a
|
|
10
|
T2in
|
CTS
|
TTL
|
n/a
|
|
11
|
T1in
|
TX
|
TTL
|
n/a
|
|
12
|
R1out
|
RX
|
TTL
|
n/a
|
|
13
|
R1in
|
RX
|
RS-232
|
2
|
|
14
|
T1out
|
TX
|
RS-232
|
3
|
|
15
|
GND
|
GND
|
0
|
5
|
Connections between MAX 232 & RS 232
In addition one can directly wire DTR (DB9 pin 4) to DSR
(DB9 pin 6) without going through any circuitry. This gives automatic (brain
dead) DSR acknowledgement of an incoming DTR signal.
Sometimes pin 6 of the MAX232 is hard wired to DCD (DB9 pin
1). This is not recommended. Pin 6 is the raw output of the voltage pump and
inverter for the -10V voltage. Drawing currents from the pin leads to a rapid
breakdown of the voltage, and as a consequence to a breakdown of the output
voltage of the two RS-232 drivers. It is better to use software which doesn't
care about DCD, but does hardware-handshaking via CTS/RTS only. The circuitry
is completed by connecting five capacitors to the IC as it follows.
|
MAX232(A) external
Capacitors:
|
|||
|
Capacitor
|
+ Pin
|
- Pin
|
Remark
|
|
C1
|
1
|
3
|
|
|
C2
|
4
|
5
|
|
|
C3
|
2
|
16
|
|
|
C4
|
GND
|
6
|
This looks
non-intuitive, but because pin 6 is
on -10V, GND gets the + connector, and not the - |
|
C5
|
16
|
GND
|
|
Drawbacks of MAX232:
i.
The MAX-232 chip receives data from the receiver,
and converts it to the standard RS-232 data format that can be read in by a
serial port on a personal computer or workstation.
ii.
For the RS-232 interface, a standard MAX232 chip is
used for level conversion. Both use the on chip USART and thus the same
firmware.
Connections In Max 232:
If you wanted to do a general RS-232 connection, you could take a bunch of long wires and solder them directly to the electronic circuits of the equipment you are using, but this tends to make a big mess and often those solder connections tend to break and other problems can develop. To deal with these issues, and to make it easier to setup or take down equipment, some standard connectors have been developed that is commonly found on most equipment using the RS-232 standards
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