ELM624
meaning of the final two bytes. If the digit is a 1, it
generally means that the final two bytes are status
bytes, while if it is a 3 the bytes represent minutes and
seconds, a 4 is days and hours.
The fourth hex digit (least significant nibble of the
second byte) generally gives system status information
such as:
on (2), no PCM (4), and mono sound (5). Each device
is likely to have its own unique interpretation of these
bits.
The second line shows that the device is still
stopped (02), that the following information will be
ss:mm (3), and that recording is not allowed due to the
tab (2). The third line states that hh:dd (4) follow. From
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.
After a short time, the following is reported by the
ELM624:
b3 - counter memory on
b2 - low battery
b1 - record tab status
b0 - invalid command received
As an example of a typical transmission, assume
that the command 1034 (VTR#0 Play) is sent. The
response would typically be:
0 6 1 2 2 4 5 8
0 6 3 2 4 9 1 1
0 6 4 2 0 1 0 0
0 2 1 2 2 4 5 0
0 2 3 2 4 9 1 1
0 2 4 2 0 1 0 0
Comparing this to the previous, one can see that
the device is now in play mode (06), with a time of
00:01:11:49. The 8 in the last position of the first row
indicates that there is a ‘memory mark’ here to show a
significant spot on the tape.
It is beyond the scope of this document to detail
the Control L standard in any more detail, but hopefully
this has been enough to get you started. Be sure to
check with your manufacturer for specifics on the
model(s) that you are using.
Many more bytes would be sent, as the time
codes are continually varying. Hitting the return key
will stop the endless stream of data and place the
ELM624 in the command state. The first line above
shows that the device is currently stopped (02), that
the last four digits will be status (1), and that recording
is not allowed due to the tab (2). The status bytes (24
and 50) provide several details such as record switch
Design Considerations
The ELM624 is an experimenter’s integrated
circuit, so it has a few peculiarities that must be
considered in any design.
expected. This simplifies the RS232 circuitry, but may
cause some confusion. Precautions should also be
taken in the circuit design to allow for the possibility
that this input may be left floating due to a
disconnected serial cable. Typically, this only requires
a large-valued resistor between the RS232 TxD pin
and Vss, as shown in the Example Application section.
As a final note, one should realize that the
ELM624’s RS232 interface does not employ any hand-
shaking signals, so the controlling computer must wait
for a response (all responses end with a carriage
return character) before issuing the next command.
Failure to do so will usually cause the IC to abort
whatever action is in progress, as the RS232 Rx signal
is given highest priority. Under keyboard control this is
unlikely to be a problem, as the character spacing is
usually adequate enough to allow the ELM624 to
complete any processing before receiving the next
ones. Under computer control, however, it is essential
that the software waits for a complete response before
proceeding with the next command.
Foremost, one should note that the LANC pin is
connected directly to the Control L bus. This presents
two areas of concern. One, if the controlled device is
powered before the ELM624, there will be a backfeed
through the (inherent) LANC pin protection diode into
the ELM624 circuitry. This can cause unexplained and
possibly erratic operation. Always power the ELM624
circuitry before connecting to the Control L bus.
The other concern related to the LANC pin is the
fact that this is a CMOS integrated circuit, and that
‘latch-up’ can occur due to induced currents from
external sources. To reduce the possibility, try to keep
connecting cables as short as possible, and consider
placing a small value resistor (100Wto 220W) in series
with this pin.
On the PC side of things, the main consideration
with the RS232 interface is the fact that the receive
signal is inverted from what would normally be
ELM624DSB
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