ELM624 Ver la hoja de datos (PDF) - Elm Electronics
Número de pieza
componentes Descripción
Fabricante
ELM624 Datasheet PDF : 16 Pages
| |||
ELM624
Talking to Your LANC Device (cont’d)
Once the controlled device has received a valid
command, it will respond with a sequence of 4 status
bytes which the ELM624 resends as a series of 8
hexadecimal digits. Although the addition of space
characters could make this data more readable, there
is not sufficient time to do this with 60Hz systems,
without losing data. The initial byte (the first two digits)
will usually be a status byte, having typical values as
follows:
01 No Tape
02 Stopped
03 Fast Forwarding 04 Recording
83 Rewinding
06 Playing
72 Stopped - tape beginning
The next nibble (third digit) is normally used to
identify the meaning of the final two bytes (the fifth to
eighth hex digits). For example, if the third digit is a 1,
it generally means that the final two bytes are status
bytes, while if the third digit is a 3 or 4, the last bytes
are time codes. A 3 might mean that the bytes are
seconds and minutes, while a 4 may mean they are
hours and days.
The fourth hex digit (least significant nibble of the
second byte) generally gives various system status
information such as (in this case):
b3- Counter memory on
b2- Low battery
b1- Record tab status
b0- Invalid command received
Often it is easier to follow an actual example to
see how the codes look. Assume that the command
1034 (VTR#0 Play) has just been sent to the ELM624.
It will format the data, transmit it the number of times
that is specified by the ATRn command, and begin
reading and sending status bytes back to the
controlling computer. The first few lines of the (lengthy)
response might look like:
02122450
02324911
02420100
Many, many more bytes would actually be sent by
the ELM624, as each successive line differs from the
previous, and the IC always transmits a line when it
differs from the previous one. Sending a single space
character to the ELM624 will stop the endless stream
of data and place the integrated circuit into the ready
state, able to receive more commands. Almost any
character will serve to stop the data stream, but the
space character is the most convenient, and the most
likely to be ignored by other devices connected to the
system. When the prompt appears, you may want to
stop the tape, if it is still running. Simply send:
>1030
Again, you’ll need to send an interrupting
character to stop the stream of status bytes. If you are
using an older computer system, there may seem to
be a delay between the typing of the space character
and the stopping of the ‘endless’ display of data on the
screen. This is not due to lack of response by the
ELM624 (it always responds within 20 milliseconds),
but is most probably that your display is not fast
enough to keep up with the data as it is received, so
be patient.
Once interrupted, the ELM624 always completes
sending the current line before returning to the ready
state, and sending a prompt character (‘>’). This
occurs very quickly in human terms, but as noted
above can take as long as 20 msec, which is very long
for computers. If you are operating your LANC device
under program/computer control, always wait until the
prompt character has been received before sending
the next command.
Returning to the status bytes that were received
above, let us analyze the response. The first line starts
with the byte ‘02’, which shows that the device is
currently stopped. The next nibble ‘1’ says that the
following nibbles will provide various status information
(the exact values depend on the controlled device).
The second line shows that the device is still
stopped (02), that the following information will be
ss:mm (3), and that recording is still not allowed due to
the tab (the 2 in the fourth position). The time code
bytes show 11 minutes and 49 seconds in this case.
The third line is similar, but states that hh:dd (4)
follow. Combining the two time response lines (the ‘3’
and ‘4’ lines), one can deduce that the counter is
currently showing 00 days, 01 hours, 11 minutes and
49 seconds. Some counters are simply a linear
reading and do not reflect the actual time duration, so
make sure of the interpretation when trying this on
your device.
If you look further down the stream of data that
you initially received when you said ‘play’ (1034), you
will see that the second hex digit changes after a
while. Typically, the received LANC bytes may have
ELM624DSD
Elm Electronics – Circuits for the Hobbyist
< http://www.elmelectronics.com/ >
10 of 16
Share Link: 