12v to 5v ttl

hello,
ive been trying to bring back old project i made that displayed data from a cars ECU to lcd screen but ive been having problems trying to interface the arduino with the new ECU. My old setup was simple since Honda ECUs use a 5v ttl and will hook up directly to the arduino. Now i am trying to use it on a Nissan ECU with their consult interface but it uses 12v ttl. copied below is the specs on the interface from PLMS protocol

1.1. Consult terminal or PC communications is via three wire bus. TX, RX and Async
Clock.
1.2. TX data to ECU level is 12V, idle low (see interface schematic). ECU output is
open collector and can be sourced from the interface logic level (5 V). The ECU
data output is idle high.
Clock to the ECU seems fine with a 5v source from the interface.
1.3. Communications baud rate is 9600 baud (default). Clock speed for this baud rate
is 153.6 Khz, plus or minus 5 khz (as tested by PL& MS). Other baud rates and
matched clocks may be possible as well. Clock GENERATED by the Host
terminal (Consult or PC Interface). Synchronizing the clock to the data lines is
NOT necessary. E.g. the clock free runs at the required speed. (16x the data rate).
1.4. The Data Format is 1 start bit (always low), 8 data bits, no parity, 1 stop bit
(always high).

currently i have been try to use this schematic also from PLMS minus the MAX232, with no luck.

What exactly doesn't work?

Do you have a scope for signal checks?

I believe I am having trouble getting the signals to something that the arduino can read. I have a few libraries that I have found for this but none of the will connect (arduino sends 3 bytes to initiate then receives a confirmation byte). I don't have a scope just a multimeter.

How are you generating the 12V? The MAX232 is the magic device which creates +/-12V from your power supply.

Given that it's in a car, 12V is readily available. You just need some MOSFETS to switch that level for you. That schematic will be a long way away from the PLMS schematic.

Can you elaborate more on that morganS? I don't need the max232 as i'm am not communicating with RS-232, all I need to do is step the signals from 12v-5v and 5v-12v so i can get the two to talk. I'm pretty much a novice with building circuits from scratch. I been reading around and finding bits of info but, I cant get a clear answer.

93DOHCZC:
I believe I am having trouble getting the signals to something that the arduino can read. I have a few libraries that I have found for this but none of the will connect (arduino sends 3 bytes to initiate then receives a confirmation byte). I don't have a scope just a multimeter.

A scope would be the simplest solution for checking dynamic signals. Without it you can only check static levels, and low (audible) frequencies using e.g. a piezo speaker/microphone. Old analog voltmeters are often more helpful than digital multimeters.

First I'd check the static level conversion. Power up your circuit, but don't connect external signals. Then check the clock generator, by e.g. listening on the last divider stage. Or connect an AC voltmeter through an capacitor to the signal, it should show roughly half (0.7) of the supply voltage. Then apply proper logic levels to all inputs, i.e. Gnd for low, and 5 or 12 V for high level. Then check the related output, whether is follows the input level (maybe logically inverted). If not, follow the signal path step by step, back to the input.

@all, do there exist useful tutorials on checking logical circuits without a scope? I've built and used a couple of simple helper circuits myself, and would like to present these in a tutorial, if they are not mentioned already somewhere else.