The Command Line Interpreter

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ELM621

ELM621DSA

Elm Electronics – Circuits for the Hobbyist

< http://www.elmelectronics.com/ >

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 (100

Ω 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.

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.