ALDL 8192 baud Hardware
There have been a number of designs published (none by GM) to enable the GM proprietory ALDL data stream to be tapped into. Our design uses a MAX232 (or MAX233) because this is the simplest way to do it. We also avoid using a separate power supply by powering the MAX chip from the PC's serial port (DTR and RTS signals must be set to +ve).
This circuit can also be used to read older ECU's 160 baud ALDL signals. Here's our circuit using the MAX232 (or MAX232A). Vehicle signals are on the left, PC signals on the right, with DB9 (and DB25 in brackets) pin assignments shown (pin N on DB9 is N/9, etc.).
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. 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. More information is available from Maxim, or you can download the data sheets for their 5 Volt Interface Products. As well, get the 78L05 data sheet from Natsemi, and the IN914A data sheet from Fairchild.
Early ECUs produced a fixed 8192 ALDL data stream when the user placed a 10k ohm resistor between ALDL connector pins A and B. Later ECUs added internal receive circuitry (the SXR Delco/Delphi transceiver chip). for enhanced capabilities, and their firmware was upgraded too. Software can be used to enable the 8192 ALDL data stream from these later ECUs.
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. The 160 baud data is sent to the PC and appears as a toggling CTS signal. thus no mechanical switching is required to select either 160 or 8192 baud data streams.
The diagnostic sense resistor R3 may not be required for later ECUs where software controls the ALDL data.
The power supply uses two signal lines (RTS and DTR) from the PC to provide a positive voltage for the MAX chip. The two diodes (D1 and D2) are used to ensure power is available even if only one signal line is positive. Capacitor C6 ensures the low power 78L05 regulator is stable in operation. Capacitor C5 provides decoupling and filtering from the PC. C1 through C4 are the charge pump and inverter capacitors - their orientation (assuming you use tantalum types) should be double checked.
early 90's era 1227808, same as a1227165 but no high speed aldl port.atc82 wrote:what ECM
a slightly modified 1977 Alfa Romeo GTVand car?
No, the 1227808 hardware only supported 160bps aldl so I wired a cp2102 TTL module to the processor's UART, so my ECM has it's own USB port.did you use the schematics posted here?
Users browsing this forum: No registered users and 13 guests