Application Note AN02 - ELM327 Circuit Construction When constructing an electronic circuit, it is often best to build it in stages, testing each as you proceed. The following pages provide typical voltages that you might expect as you build the ELM327 Example Application circuit, first with no integrated circuits installed, and then with all in place. Hopefully, it will help you to locate wiring issues before they cause problems. First, wire everything to these sockets, but do not install the ICs. Complete the rest of the circuit, power it with a small 9V ‘radio’ battery, and then measure the voltages in your circuit. Compare your voltages to those shown in red on Figure 2. If there are any significant discrepancies, find out why before proceeding. Note that a 9V battery is chosen as it will not be able to supply very large currents, so if there are serious problems, there will be little or no damage. Once you are satisfied that your circuit will not damage the integrated circuits, install them in their sockets, and apply a source of about 12V to the circuit. You should then see voltages similar to those of Figure 3. Once satisfied that the voltages are correct, you can begin testing with the software of your choice. Do not connect to the vehicle when testing the software, as you are only testing the communications at that point. Once the software is able to communicate with the interface, test each protocol in sequence, with no vehicle connected. Use the SP command to select individual protocols, and then send 0100. If all goes well, you should only see ‘NO DATA’s or ‘UNABLE TO CONNECT’ and no messages that indicate a problem with the vehicle such of ‘BUS BUSY’ or ‘FB ERROR’. If you do see either of these latter two messages, then there is a problem with your wiring – correct it before connecting to a vehicle. When you’ve finished the above, and all appears to be working well, you may connect to a real vehicle and begin testing the complete ‘package’. Hopefully all will work well for many years. Testing the ELM327 IC Although we do not advise it, we often get emails from people that are trying to test their ELM327 before installing it into the final circuit. This exposes the chip to conditions that may not be good for it, as people often report open-circuited inputs, incorrect supply levels, etc. CMOS integrated circuits should never have an input left open-circuited - it should always be connected to either 0V or 5V, else erratic behaviour and possibly damage to the IC might occur. If you insist on taking the IC from its protective tube and installing it into a test circuit, use the one in Figure 1. If the IC is able to initialize itself internally and start functioning, you will see the four LEDs light in sequence, followed by the one on pin 26 lighting one more time. Constructing Figure 9 from the Data Sheet Most people build the Example Application circuit that we show in the data sheet (Figure 9), so we will discuss it here. When you do begin to build this circuit, we advise using sockets for the MCP2551 and ELM327 ICs, and building the circuit in two stages. +5V 4x 470 Ω 78L05 + 7 to 20V - +5V 28 0.1µF 27 26 25 24 23 22 0.1µF 21 20 19 18 17 16 15 9 10 11 12 13 14 327 +5V 1 2 3 4 5 6 7 8 4.00MHz Figure 1. A minimal ELM327 test circuit 27pF AN02 rev A Elm Electronics – Circuits for the Hobbyist www.elmelectronics.com 27pF 1 of 3 Application Note AN02 - ELM327 Circuit Construction CAN-L 0.00 14 0.00 6 CAN-H R32 R33 100Ω 100Ω 0.00 9.24 0.00 8 R31 4.7KΩ 0.00 C8 560pF 0.00 1 +5V R19 510Ω 16 Battery Positive 5 2 3 0.00 78L05 -0.57 0.00 +5V 4.36 R22 10KΩ pin 12 R21 22KΩ 3.53 4.36 9.24 28 26 25 24 2 3 4 5 6.10 4.88 Q2 R4 470Ω 6.10 6.10 R7 4.7KΩ Q1 19 18 17 7 8 9 10 11 12 13 to R21 0.00 2 (RxD) 0.00 1 (DCD) 5.36 4 (DTR) 6 (DSR) 14 C3 27pF 0.00 2.42 R6 4.7KΩ * see text, page 55 0.00 D3 ELM327 pin 14 0.00 5.49 to R15 8 (CTS) 0.00 2.90 C4 27pF Notes: - The MCP2551 and ELM327 have NOT been installed 0.00 R10 22KΩ R11 10KΩ 0.00 Q3 R23 4.7KΩ U1 0.00 R14 4.7KΩ R12 100KΩ - 9V dry cell used to power the circuit (ie for Battery Positive) - A PC is connected to the RS232 Interface, and terminal software is running R9 10KΩ R15 4.7KΩ 5 (SG) 0.1µF -2.61 15 0.00 R36* 22KΩ 10 J1850 Bus - 20 X1 4.00MHz 0.00 2 J1850 Bus + 21 R3 470Ω R8 10KΩ AN02 rev A 6 R2 470Ω 317L R5 240Ω D2 22 0.00 8.51+12V 2.42 U4 0.00 -5.18 7 (RTS) C2 0.1µF (J1962) 23 n.c. 16 327 NOT installed 1 R35 10KΩ 27 +5V 1.64 OBD Interface +5V +5V 3.53 R34 47KΩ Q8 L1-L4 R27-30 470Ω Vbat +5V 3 (TxD) D4 C5 2.90 ELM327 R20 47KΩ (DB9F) R24 10KΩ -5.76 R25 10KΩ D5 0.00 RS232 Interface Q9 0.00 Q6 Q7 2.97 R1 470Ω R26 4.7KΩ 4.96 R18 2.2KΩ C6 0.1µF +5V 0.00 9.18 L5 PWR C1 0.1µF 0.00 R16 2.2KΩ 4.96 D1 R17 510Ω 9.24 +5V U3 5 Signal Ground 4 C7 0.1µF Vbat 15 ISO-L 7 ISO-K 6 MCP2551 NOT installed U2 C9 560pF Vbat 7 +12V 8.51 Vbat +5V - No connections were made to the OBD (J1962) interface 4.96 Q4 0.00 Q5 R13 10KΩ Elm Electronics – Circuits for the Hobbyist www.elmelectronics.com Figure 2. First stage wiring tests 2 of 3 Application Note AN02 - ELM327 Circuit Construction CAN-L 2.56 14 2.48 6 CAN-H R32 R33 100Ω 100Ω 2.48 12.05 1.73 8 R31 4.7KΩ 2.56 C8 560pF 2.48 1 +5V R19 510Ω 2 16 Battery Positive 5 3 U2 78L05 4.96 C1 0.1µF 4.96 11.96 -0.57 0.00 +5V 4.96 R22 10KΩ pin 12 R21 22KΩ 4.95 4.96 12.05 28 27 26 25 24 1 2 3 4 5 3.69 Q2 R4 470Ω 4.92 4.92 R7 4.7KΩ Q1 18 17 7 8 9 10 11 12 13 to R21 0.00 2 (RxD) 0.00 1 (DCD) 5.36 4 (DTR) 6 (DSR) 14 C3 27pF 0.00 1.24 R6 4.7KΩ * see text, page 55 0.09 D3 ELM327 pin 14 0.00 5.49 to R15 8 (CTS) 0.00 3.79 C4 27pF Notes: - Bench supply used for Battery Positive (measured as 12.05V) 2.29 R10 22KΩ R11 10KΩ 0.09 Q3 R23 4.7KΩ U1 0.00 R14 4.7KΩ R12 100KΩ - Measured current draw for this entire circuit is about 33 mA - A PC is connected to the RS232 Interface, and terminal software is running R9 10KΩ R15 4.7KΩ 5 (SG) 0.1µF -2.61 15 0.00 R36* 22KΩ 10 J1850 Bus - 19 X1 4.00MHz 0.09 2 J1850 Bus + 20 R3 470Ω R8 10KΩ AN02 rev A 6 R2 470Ω 317L R5 240Ω D2 21 1.94 4.92 0.00 -5.18 7 (RTS) 11.33 +12V U4 0.05 (J1962) 22 n.c. 16 327 C2 0.1µF OBD Interface 23 +5V 2.14 R35 10KΩ +5V +5V 4.95 R34 47KΩ Q8 L1-L4 R27-30 470Ω Vbat +5V 3 (TxD) D4 C5 3.79 ELM327 R20 47KΩ (DB9F) R24 10KΩ -5.76 R25 10KΩ D5 0.00 RS232 Interface Q9 0.00 Q6 Q7 2.97 R1 470Ω R26 4.7KΩ 4.96 R18 2.2KΩ C6 0.1µF +5V 0.00 R16 2.2KΩ L5 PWR D1 R17 510Ω 12.05 +5V 4.96 5 Signal Ground 4 C7 0.1µF Vbat 15 ISO-L 7 ISO-K 6 MCP2551 C9 560pF Vbat 7 +12V 11.33 U3 Vbat +5V - No connections were made to the OBD (J1962) interface 4.96 Q4 0.00 Q5 R13 10KΩ Elm Electronics – Circuits for the Hobbyist www.elmelectronics.com Figure 3. Final Wiring Tests 3 of 3