ELM621 데이터 시트보기 (PDF) - Elm Electronics
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ELM621 Datasheet PDF : 6 Pages
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ELM621
Design Considerations
The ELM621 makes it easy to quickly construct
a control circuit, but may be frustrating if you don’t
keep a few things in mind. Here are some pointers to
consider…
When two power supplies are involved, always
try to have the ELM621 powered at either the same
time, or before applying power to the other circuitry.
Failing to do so may allow backfeed through the
(inherent) port pin protection diode, causing
unexplained and possibly erratic operation.
Keep in mind that this is a CMOS integrated
circuit, and that ‘latch-up’ of the chip can thus occur.
To reduce the possibility, keep connecting cables as
short as possible, and consider placing a small value
resistor (100Ω to 220Ω) in series with the port pins.
The RS232 interface is not complicated, but
consideration must be given to the fact that the
receive signal is inverted from what would normally
be expected. This simplifies the RS232 interface
circuitry, but may result in some confusion. Refer to
the Example Application section for a typical method
of connection.
Under high-speed computer control, problems
may result due to the fact that there are no hand-
shaking signals provided. This is generally not a
problem if controlled by keyboard entry, as the
ELM621 is considerably faster than a human typist,
but under software control, one should always wait
for the prompt (‘>’) character before issuing another
command.
As a final note, it should be realized that linefeed
characters are not sent by the ELM621 with the
carriage returns. If viewing the output on a terminal,
many users may wish to have their terminal program
add the linefeeds in order to improve readability.
The Command Line Interpreter
All commands are transmitted to the ELM621
one line at a time. A line consists of the character ‘A’
followed immediately by the letter ‘T’, then up to 16
control characters, and a terminating carriage return
character (0x0D). The carriage return serves as an
execute command, the line not being translated and
executed until this character is received.
The 16 byte command buffer is implemented as
a ‘first-in first-out’ (FIFO) type, retaining only the
most recent 16 characters since the last AT. Any
other characters are lost (there is no scroll-back
capability). The buffer is not cleared immediately
after executing a command line. This allows for a
‘command line repeat’ mode if a carriage return is
received immediately after executing a line. Receipt
of any other character will clear the buffer. This
feature is particularly useful when simply toggling a
bit on and off, or stringing two toggles together to
form a pulse in order to trigger another circuit.
A command line may contain spaces or commas
between the individual commands, as these do not
affect operation. Often when using a terminal, it is
convenient to separate commands with one of these
characters in order to improve readability, but keep
in mind that all two-character commands must not
have a comma or space between the two
characters.
One other feature of the interpreter is that it is
not case-sensitive. That is that it is able to accept
either upper or lower case characters in the input,
and they are treated equally (an ‘A’ is treated the
same as ‘a’, ‘T’ is the same as ‘t’, etc.)
Error reporting by the ELM621 is quite minimal.
Checking of a line for errors does not occur until
after the carriage return is received, and any error in
the input is reported with a simple question mark. If
there is any error detected in the syntax of a line,
none of the line is executed.
Occasionally there may be delays in the
transmission from the controlling device to the
ELM621, either due to the operator pausing, or to a
break in the communications channel. When a
pause of greater than 20 seconds occurs, the
ELM621 will time out and automatically return to its
‘waiting for AT’ state.
ELM621DSA
Elm Electronics – Circuits for the Hobbyist
< http://www.elmelectronics.com/ >
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