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The SAE J1962 standard dictates that all OBD drops of two diodes and an LED) with respect to
compliant vehicles must provide a standard connector vehicle common. Had the input been directly
near the driver s seat, the shape and pinout of which is connected to pin 4, the threshold would have been
shown in Figure 3 below. The circuitry described here approximately 5 volts - much higher than the 3.5 volts
will be used to connect to this plug without modification specified by the standard.
to your vehicle. A very basic RS232 interface is shown connected
The male J1962 connector required to mate with a to pins 5 and 6 of the ELM322. This circuit steals
vehicle s connector may be difficult to obtain in some power from the host computer in order to provide a full
locations, and you could be tempted to improvise by swing of the RS232 voltages, without the need for a
making your own connections to the back of your negative supply. The RS232 pin connections shown
vehicle s connector. If doing so, we recommend that are for a 25 pin connector. If you are using a 9 pin, the
you do nothing which would compromise the integrity connections would be 2(RxD), 5(SG) and 3(TxD).
of your vehicle s OBD network. The use of any RS232 data from the computer is directly
connector which could easily short pins (such as an connected to pin 5 of the IC through only a 47K&!
RJ11 type telephone connector) would definitely not current limiting resistor. This resistor allows for voltage
be recommended. swings in excess of the supply levels while preventing
The circuit of Figure 4 on the next page shows damage to the ELM322. A single 100K&! resistor is
how the ELM322 would typically be used. Circuit also shown in this circuit so that pin 5 is not left floating
power is obtained from the vehicle (OBD pins 16 and if the computer is disconnected.
5) and, after some minor filtering, is presented to a five Transmission of RS232 data is via the single PNP
volt regulator. Notice that the common point of the transistor connected to pin 6. This transistor allows the
regulator is returned to vehicle ground through a diode output voltage to swing between +5V and the negative
and an LED, effectively raising the circuit common voltage stored on the 0.1µF capacitor (which is
about 2.5 to 3 volts above that of the vehicle. This charged by the computer s TxD line). Although it is a
gives a net 7.5 to 8 volt positive supply for the OBD simple connection, it is quite effective for this type of
bus, as required by the standard (the ground signal application.
shown throughout the schematic refers to the circuit Finally, the crystal shown connected between pins
common and not the vehicle s chassis ground). 2 and 3 is a common television type that can be easily
Note that by offsetting the regulator in this way, and inexpensively obtained. The 27pF crystal loading
the LED and the 750&! resistor (which provides the capacitors shown are typical only, and you may have
current for the LED) become critical components that to select other values depending on what is specified
must not be eliminated. Also, one other subtle result of for the crystal you obtain.
this is that one must take care not to connect the This completes the description of the circuit. While
vehicle s common to the computer s common, as the it is the minimum required to talk to an OBD equipped
LED will be shorted out, reducing the supply to 5 volts vehicle, it is still a fully functional circuit. As an
which is below the required level. experimenter, you may want to expand on it, providing
The remaining connection to the OBD bus (pin 2) more protection from faults and electrostatic
is the data line required for communications. Data is discharge, or providing a different interface for the
transmitted onto the bus from the ELM322 via the PNP RS232 connection to the computer. Then perhaps a
transistor, the diode, and the 100&! current limiting Basic program to make it easier to talk to the vehicle,
resistor (which also provides moderate waveshaping). and a method to log your findings, and&
The diode is needed to protect the circuitry from
currents which could flow through the transistor due to
the higher voltages on the bus. Note that the 10K&!
1 8
pulldown (loading) resistor returns to vehicle common,
providing the data bus with a full (7.5V) voltage swing. 9 16
Data is received from the OBD bus and level
shifted by the NPN transistor shown connected to pin
4 of the ELM322. Using a transistor this way forces the Figure 3. Vehicle Connector
logic transition point to be at about 3V (the voltage
ELM322DSC Elm Electronics Circuits for the Hobbyist 15 of 16
ELM322
OBD
Interface
Notes: - NPN transistor is a
16 78L05 +5V
2N3904 or similar
(Battery
- PNP transistors are
Positive)
0.47µF 750&!
2N3906 or similar
Power On
LED
- Diodes are 1N4148,
5
or 1N4001, etc.
(Vehicle
Ground)
10K&!
+5V
4.7K&!
100&!
2
(Bus +)
+5V
0.01µF
+5V
1 8
27pF
3.58MHz
RS232
2 7
Interface
3 6
27pF 10K&!
4 5
3 (RxD)
+5V
4.7K&!
0.1µF
7 (SG)
4.7K&!
47K&!
10K&!
2 (TxD)
100K&!
Figure 4. Typical OBD to RS232 Interface
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