ELM624
Example Application
Figure 1 shows how the ELM624 would typically
be connected in a circuit. A very basic RS232 interface
Circuit timing is derived from the 3.58MHz
reference crystal that is connected to XT1 and XT2
(pins 2 and 3) as shown. These crystals are quite
common television types that should not be difficult to
obtain.
is provided, and the Control
unbuffered.
L
connection is
There are two types of Control L or LANC
connectors commonly used. One is the mini-DIN type
connector (similar to S-VHS or Apple serial port
connectors), and the other is the 2.5mm stereo plug
type. The mini-DIN is shown below, with a 100W
resistor in series. As previously mentioned, the resistor
is recommended for helping to prevent latch-up and
also for protection of the ELM624 from direct shorts to
the supply (most Control L connectors provide DC on
one of the pins). If a 2.5mm stereo plug is used, the tip
is generally the LANC signal, the small ring is often a
DC supply, and the sleeve is the circuit common or
VSS.
Note that no additional pullup resistance is used in
this circuit from pin 7 to VDD, as the ELM624’s internal
resistance is often sufficient when coupled with pullup
resistors in the controlled device. Occasionally this
may not be sufficient to provide a full logic swing,
however, and an additional resistor (typically 4.7KW)
may be needed.
For connecting to the computer, a simple RS232
interface is used. It is not as elaborate as some, but is
sufficient for this purpose. Generally, the RS232
interface is operated in half-duplex mode, with the
ELM624 listening to the computer, and then the
computer waiting for a response. Since both devices
are not likely to be ‘talking’ at the same time, we are
able to ‘steal’ a bias voltage from the computer for the
ELM624 circuitry to use, as shown. A diode provides a
return negative from the TxD line that the RxD can
use, while a small capacitor provides charge storage.
The capacitor size shown should be sufficient in most
cases, but if you have trouble with characters being
echoed, etc. you might want to increase its size to
several µF. The RS232 pin connections shown are for
a 25 pin connector. If you are using a 9 pin, the
connections would be 2(RxD), 5(SG) and 3(TxD).
That should get you started with the interface.
Perhaps with a little programming…
+5V
4
5
2N3906
2
3
100W
1
Control L
mini DIN
3 (RxD)
4.7KW
+5V
0.1µF
0.1µF
RS232
DB25F
10KW
7 (SG)
Connector
1
2
3
4
8
7
6
5
1N4148 or
similar
3.58MHz
27pF
2 (TxD)
33KW
100KW
27pF
Figure 1. Control L (LANC) to RS232 Interface
ELM624DSB
Elm Electronics – Circuits for the Hobbyist
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