Help identifying current limit?

Thread Starter


Joined Feb 6, 2016
Hi everyone,

I'm new here; I decided to sign up and post my question as it looks like there are some very knowledgeable and helpful people hanging out here. If my question is inappropriate somehow, please just let me know, being new I'm not used to your expectations.

Just a touch of background to paint the broader picture around my question a little. I've been working on a little hobby project with my 1987 General Motors ECM (simple computer based on Motorola 680x microprocessor). I have retrofitted a larger PROM in order to be able to change the code. I have added a dual port 1K SRAM chip to the address/data bus of the ECM. In my modified PROM code, I wrote an interrupt routine that dumps the ECM RAM plus a checksum to one port of the dual port SRAM I added. To the other port, I connected an Arduino Mega which triggers the ECM interrupt, reads the data, and does all kinds of fun stuff from there. I am powering the SRAM chip from the ECM's Vcc, the Arduino is powered completely separately (regulator plugged in at the same place the ECM gets 12+volt in and ground to avoid ground loop).

Generally everything works. The trouble is that, with the engine running, every once in a while (less than 1%) when I read the data from the SRAM chip to the Arduino, the checksum doesn't match. 99% of the time if I just re-run my read routine, without retriggering the interrupt the read is error free. I've tried many things over time, adding 0.1uf ceramic disc capacitors between Vcc and ground on my IC's, changing my data cables, adding a 74HCT245 buffer, nothing seems to help. I'm almost ready to just rely on my re-read error correction, but I'm worried about one thing -- with the IC's I have added to the ECM, could I be exceeding the maximum allowable current for the circuit? That's haunting me because in that case the ECM is bound to malfunction once in a while too!

So here is my question - is it reasonable to ask you guys to have a look at the top right of this circuit and advise me as to whether or not the power rectifier portion limits the maximum allowable current at Vcc?

I tried to figure it out from similar transistor based rectifier circuits, but unfortunately my knowledge of circuits isn't nearly strong enough.

Unfortunately I don't have specs on those transistors. They were made for GM and have no decypherable markings on them. This is the best and most detailed schematic that I believe exists publicly.

Thank you all very much for reading and considering my question!