Capacitor C6 ensures the low power 78L05 regulator is stable in operation.
The two diodes (D1 and D2) are used to ensure power is available even if only one signal line is positive. The power supply uses two signal lines (RTS and DTR) from the PC to provide a positive voltage for the MAX chip.
The diagnostic sense resistor R3 may not be required for later ECUs where software controls the ALDL data. thus no mechanical switching is required to select either 160 or 8192 baud data streams. The 160 baud data is sent to the PC and appears as a toggling CTS signal. The 8192 baud Rx and Tx data to/from the PC is combined with a diode and resistor (D3, R1) before being sent to the ECU. Software can be used to enable the 8192 ALDL data stream from these later ECUs. for enhanced capabilities, and their firmware was upgraded too. Later ECUs added internal receive circuitry (the SXR Delco/Delphi transceiver chip).
As well, get the 78L05 data sheet from Natsemi, and the IN914A data sheet from Fairchild.Įarly ECUs produced a fixed 8192 ALDL data stream when the user placed a 10k ohm resistor between ALDL connector pins A and B. More information is available from Maxim, or you can download the data sheets for their 5 Volt Interface Products. If you use a MAX233, which has a different pinout to the MAX232, you don't need to use any of the capacitors C1-C4, but the regulator still requires C5 and C6. Note that the 3.3 uF capacitors (C1-C4) may be reduced to 1 uF if these are readily at hand, or to 0.1 uF if you use the MAX232A.