TECHNICAL NOTES ON THE EEC-IV MCU - Auto diagnostics

Eectch98-Part4.fm
put the next byte of data on the MB bus (if the MEMORY is driving the MB Bus).
• IT can be thought of as the "Instruction Transfer" line. What it really does is
choose whether the 8061 is accessing the SPC (if IT is high) or the DAR (if IT is
low).
• DI can be thought of as the "Direction Indicator" for the MB bus. If DI is high,
then memory has the rights to the MB bus. If DI is low then the processor has
the rights to the MB Bus.
(DAR - data address registerSPC - slave program counter)
An example would be reading a memory location using the DAR. In a "NORMAL" memory
system (one with an address and a data bus) if we were going to read a memory location
we would:
1. Place the address on the address bus.
2. Output the memory "read enable" to "enable" the memory chip.
3. Read the information off of the "DATA" bus.
On an EEC MB bus system we would follow the same pattern:
1. Write the DAR (give the memory’s address register the address).
a. Lower DI - this gives the rights to the MB bus to the 8061.
b. Lower IT - this tells the memory system we’re using the DAR, not the SPC.
c. Drive MB bus with low 8 bits of the address.
d. Bring /STB low for a cycle - tells memory to latch the low address.
e. Drive MB bus with high 8 bits of the address.
f. Bring /STB low for a cycle - tells memory to latch the high address.
At this point the memory system has our "ADDRESS" that we want to read.
7. Raise DI - This tells the memory system that it has rights to the MB bus.
This is the same as sending a "memory enable" or "memory read" signal.
8. Read the data off of the MB bus.
a. Read the low byte off the MB bus.
b. Bring /STB low for a cycle.
c. Read the high byte off the MB bus.
One note: The EEC uses a "16 bit" memory, so an address didn’t have to be supplied
for the 2nd 8 bits of data, but when using the DAR you must begin reading from an
even address, and the odd address will be available the following /STB. If you use
the SPC to read the memory, you can (in theory) just keep strobing and read the
entire chip. I’ve found that I can only "strobe" about 300 bytes of data out of the
memory before it "clams up". Your results may vary.
MEMORY CONTROL FUNCTIONS
DI IT /STB
0 0 1 CPU SENDING ADDRESS TO DAR
0 1 1 CPU SENDING ADDRESS TO SPC
1 0 1 MEMORY SENDING DATA POINTED TO BY DAR TO CPU
1 1 1 MEMORY SENDING DATA POINTED TO BY SPC TO CPU (USUALLY AN INSTRUCTION)
0 0 S CPU TELLING MEMORY TO "LATCH" DATA INTO DAR
0 1 S CPU TELLING MEMORY TO "LATCH" DATA INTO SPC (PROGRAM JUMP)
1 0 S CPU TELLING MEMORY TO "BRING UP" HIGH BYTE