Oct 2010
Many years ago when I was really into shortwave listening and
mediumwave DXing I bought an ICOM R71a communications receiver. It's
an excellent receiver but with generally poor-sounding audio. I never
could understand how an otherwise excellent radio could end up with
such bad sound; maybe there's an interesting story there. However,
what caught my eye and raised a red flag about an obvious failure mode
was the use of a battery backed up static RAM module.
When the radio was designed in the early 80's flash memory wasn't
available so the designers created something similar: A daughter board
with a static RAM chip, some support circuitry, and a lithium cell.
When the unit is powered off that cell supplies the necessary voltage
to keep the static RAM contents alive. Such a module can operate for
many years because an idle static RAM takes very little
current, but eventually the cell will run down.
Various sources speculated that the static RAM held the radio's
operating software, which meant that its failure would be a BAD THING.
It's not true, though: The module only has the unit's operating
configuration as well as the contents of VFO A/B and the 32 memories.
It can still be fatal to lose the contents but for some reason it
doesn't feel as if it's a total loss even though the module still has
to go in for repair.
After owning the radio for a few years I became nervous fearing that
the lithium cell could run down anytime. This gave me a sense of
urgency about backing up the RAM contents. I reverse-engineered the
circuit board and used a digital designer to hack together a circuit
to read each location. It's not too irritating to clock the address
bus manually on a 1k x 4 static RAM and read each location
individually, especially when most are empty because I had emptied the
32 memories first.
Finally I had a backup of the RAM and so I was comfortable waiting for
the cell to run down. The joke is on me thought: Fifteen years after
doing that work the cell is still good! It's now 21 years old and
measures 3.4 volts with the module out-of-circuit.
Interface
Here are the RAM module pin definitions for the benefit of those without a
schematic. Note how the pinout looks like the typical interface for a RAM
IC; not surprising since the module is directly connected to the memory
bus.
Only two pins need clarification. The R71a's AD10 line is used as a
Chip Enable line that's active low. The second is a line I've
labelled *WP which is not connected to the R71a but
reverse-engineering the RAM module board shows that it's a Write
Protect line that is active low (with onboard pull-down); the first
256 memory locations are write protected unless this line is connected
to +5V. In other words, those locations are read-only from the R71a's
point of view because this pin is not connected.
Adding write protect circuitry required a few components; the non-zero
cost might have been objected to by bean-counters, so I wonder if
there was a justifiable need for it. Did pre-production versions of
the R71a or other products that used the same module sometimes go
insane and overwrite the RAM, blowing away the unit's config and
bricking it? Or was the possibility of this enough justification?
Was there ever any report of memories being lost or garbled? I've
never had a RAM module problem with my R71a.
Contents
What follows is a hexdump of the RAM contents, under these conditions:
- VFO A = 0100.0 AM
- VFO B = 0100.0 AM
- All memories cleared
000: 0 0 F 1 F F 1 F F 0 0 2 F F F F 200: F F F F F F F F F F F F F F F F
010: 0 0 0 B 5 9 6 8 6 8 0 0 0 3 2 0 210: F F F F F F F F F F F F F F F F
020: 0 5 9 3 0 0 0 5 0 0 0 0 0 0 0 0 220: F F F F F F F F F F F F F F F F
030: 0 1 0 0 0 0 0 0 3 0 0 0 3 0 0 0 230: F F F F F F F F F F F F F F F F
040: 1 0 0 0 0 0 0 0 3 0 0 F F F F F 240: F F F F F F F F F F F F F F F F
050: F F F F F F F F F F F F F F F F 250: F F F F F F F F F F F F F F F F
060: F F F F F F F F F F F F F F F F 260: F F F F F F F F F F F F F F F F
070: F F F F F F F F F F F F F F F F 270: F F F F F F F F F F F F F F F F
080: F F F F F F F F F F F F F F F F 280: F F F F F F F F F F F F F F F F
090: F F F F F F F F F F F F F F F F 290: F F F F F F F F F F F F F F F F
0A0: F F F F F F F F F F F F F F F F 2A0: F F F F F F F F F F F F F F F F
0B0: F F F F F F F F F F F F F F F F 2B0: F F F F F F F F F F F F F F F F
0C0: F F F F F F F F F F F F F F F F 2C0: F F F F F F F F F F F F F F F F
0D0: F F F F F F F F F F F F F F F F 2D0: F F F F F F F F F F F F F F F F
0E0: F F F F F F F F F F F F F F F F 2E0: F F F F F F F F F F F F F F F F
0F0: F F F F F F F F F F F F F F F F 2F0: F F F F F F F F F F F F F F F F
100: F F F F F F F F F F F F F F F F 300: F F F F F F F F F F F F F F F F
110: F F 0 0 0 F 1 0 0 0 0 0 0 0 0 0 310: F F F F F F F F F F F F F F F F
120: 0 0 0 0 0 1 7 0 0 0 0 0 0 0 0 0 320: F F F F F F F F F F F F F F F F
130: 0 0 2 0 5 1 0 1 0 0 0 0 0 0 0 0 330: F F F F F F F F F F F F F F F F
140: 0 0 0 0 2 0 5 1 0 1 0 0 0 0 F F 340: F F F F F F F F F F F F F F F F
150: F F F F F F F F F F F F F F F F 350: F F F F F F F F F F F F F F F F
160: F F F F F F F F F F F F F F F F 360: F F F F F F F F F F F F F F F F
170: F F F F F F F F F F F F F F F F 370: F F F F F F F F F F F F F F F F
180: F F F F F F F F F F F F F F F F 380: F F F F F F F F F F F F F F F F
190: F F F F F F F F F F F F F F F F 390: F F F F F F F F F F F F F F F F
1A0: F F F F F F F F F F F F F F F F 3A0: F F F F F F F F F F F F F F F F
1B0: F F F F F F F F F F F F F F F F 3B0: F F F F F F F F F F F F F F F F
1C0: F F F F F F F F F F F F F F F F 3C0: F F F F F F F F F F F F F F 0 0
1D0: F F F F F F F F F F F F F F F F 3D0: 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
1E0: F F F F F F F F F F F F F F F F 3E0: 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
1F0: F F F F F F F F F F F F F F F F 3F0: F F F 0 F F F F F F F F F F F F
