Logic question (answered)

Thread Starter

metermannd

Joined Oct 25, 2020
210
So, I have all the wrinkles smoothed out of my attempt at refreshing the design of this old piece of test equipment, save one.

If you look at the schematic and the dotted box in particular, you'll see a line that goes nowhere.

This went to pin 5 of an MC14044 quad R-S latch, and that IC and the external pulse input section are not populated in the version I'm copying. According to the MC14044 data sheet: When pin 5 (which the stray line otherwise goes to) is low, the four latch outputs (tied to D0-D3) go into tri-state mode - that means the latch 'disappears' from the bus, right?

When pin 5 goes high, then the latches collect pulses from whatever may be connected on the four R-S input pairs.

My question is... since the MC14044 is not populated on the board, can I then just take out U7B, U16A, and U9B? (meaning, disconnect the output, tie the inputs to ground).

It would be an absolute no-brainer YES, EXCEPT for U7B!!!

As you can see, it takes the decoded address and ties it into D7 on the data bus?

The OTHER head-scratcher relating to U7B is the input marked "J4-5". From that gate, it simply heads off-board to another section, which is NOT populated, leaving that pin FLOATING (which, I learned in another thread I posted a while back, is a design 'tsk-tsk'). I've checked that board several times and that's how that section is.

So, I'm hoping the brain trust can see something that I'm missing.

If I can get this last riddle solved, I can then either start ordering parts to build this thing or I can start looking at using this as a baseline for Arduino as was suggested AGES ago.

bus question.png

PS: Much appreciation to those who have patiently answered my questions or otherwise put up with my presence so far.
 

KeithWalker

Joined Jul 10, 2017
1,912
You forgot to mention what the old piece of test equipment is and what it does. That's probably why this request got 50 views but no replies.
 

Thread Starter

metermannd

Joined Oct 25, 2020
210
Address ranges based on firmware dump:

0x0000-0x000F Control registers (MC6821, MC6840, etc)
0x0080-0x00FF RAM (MC6810)
0x0C00-0x0C7F NVRAM (HM-6561 - on second board; just beyond J6 / J8)
0x0CF8-0x0CFF Function select (74LS138 - also on second board beyond J6 / J8)
0xE000-0xEFFF ROM (U11)
0xF000-0xFFFF ROM (U17)
 

Thread Starter

metermannd

Joined Oct 25, 2020
210
You forgot to mention what the old piece of test equipment is and what it does. That's probably why this request got 50 views but no replies.
It's part of a powerline carrier system - this board set could be populated for use in a 'load management terminal', 'distribution control terminal', or 'field configuration unit'. I'm working on replicating the 'field configuration unit', mainly to see how it works.
 

Thread Starter

metermannd

Joined Oct 25, 2020
210
I worked out that the address of the latch IC in question is 0x0008.

I recently got my hands on a tool that would allow me to do proper disassembly of the firmware used for both the LMT and FCT versions of this logic board. The LMT code has two distinct calls to location 0x0008 (reading the current state of the 4044 latch IC), whereas the FCT code does NOT contain any such calls. So that validates Mr. Chips' assessment, and I can eliminate that part at long last. :)
 
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