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Radio Intercept R&S GA-082 ← Programs
Memory arrangement
This page is part of our description of the GA-082, an
FSK Analyzer for telegraphy signals, introduced in 1983 by
Rohde & Schwarz (R&S) in München (Germany).
The memory map of this device is described below, in particular in
relation to the RAM and ROM banks.
The diagram below shows the physical arrangement of the memory
devices, as well as their slot name and part identifier.
➤ Download EPROM contents
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The Intel 8086 microprocessor has 20 address lines (A0-A19), which is good
for 1MB of memory space. In the GA-082, only the lower 15 address
lines are used (A0-A14), which gives us 32KB of address space.
The highest two of these address lines (A13-A14) are fed to two
74LS138 address decoders, resulting in
4 individual memory banks (0-3) of 8KB each.
As the 8086 has a 16-bit wide data bus,
and the memory devices are all 8-bit, two memory devices are needed for each
bank, whilst the lowest address line (A0) is used to select between them
(i.e.odd/even bytes).
The diagram above shows a simplified circuit diagram of the address
decoder, which consists of two 74LS138 devices. It decodes address lines
A13 and A14 (A-B) into four banks (0-3). The one at the right selects
the lower 8 data bits (MLn), whilst the left one is for the
upper 8 bits (MHn).
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- RAM
- Expansion ROM
- Firmware lower bank
- Firmware upper bank
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The diagram below shows the memory map of the GA-082.
Bank 0 (0x0000-0x0FFF) holds the Random Access Memory (RAM).
It must be at the lowest addresses as it contains the interrupt vectors.
It consists of two
HM6116 devices of 2KB each. This means that the total
RAM space is 4KB. As address line A12 is not connected to the RAMs,
this 4KB is mapped twice in the available 8KB space. In other words:
the RAM can be addressed as 0x0000-0x0FFF or 0x1000-0x1FFF.
Banks 2 and 3 (0x4000-0x7FFF) are used for the firmware,
which consist of four 2732 EPROMs of 4KB each (C, D, E and F).
At the top of the ROM-space (at address 0x7FF0) is the start address
of the firmware. The 8086 microprocessor interrogates this memory location
on startup and then jumps to the specified address.
The entire memory map is repeated every 0x8000 bytes.
Bank 1 (0x2000-0x3FFF) is used for (optional) user programs, for which
two empty sockets are available on the processor board.
Each socket accepts a 2732 EPROM (4KB), which means that 8KB of memory
is available for user programs. The two empty sockets are wired in parallel
to connector X16 on the processor board, which in turn is wired the the
DC37 socket at the rear of the case. This allows
user programs to be installed as a cartridge as well as internal EPROMs.
Note that internal and external expansion is mutually exclusive.
It is not possible to use both expansion methods simultaneously.
In other words: it is not allowed to install an external cartridge
when the two expansion sockets (D17 and D18) on the processor board are
occupied.
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As the above expansion method is rather limited — it allows only one
user program cartridge to be installed — R&S developed a more
versatile solution in the form of the
B1 expansion unit (GA-082/B1).
It is connected to the same DC37 expansion connector as the
single-program cartridge (A1) described above.
The B1 unit has slots for up to 45 program cartridges, each of which
can be mapped into the 8KB space (A, B) from 0x2000 to 0x3FFF.
There is a problem however.
The DC37 connector carries only 12 address lines (A1-A12) plus the
ML1 and MH1 signals (based on A0).
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This means that the maximum amount of memory that can be
addressed is 8KB, and that by default there is no mechanism to select
the desired cartridge.
Although we currently don't know how this was solved, it seems likely
that some kind of bank switching was implemented in the B1,
by writing to a a reserved memory location. The RTC of the B1
is also addressed this way.
Confusingly, the B2 program cartridges of the
B1 expansion unit contain two 2764 EPROMs of 8KB each,
which means that each cartridge holds no less than 16KB of memory.
How is this solved?
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Short answer: it isn't. Address line A12 of the 2764 is tied to +5V,
which means that the lower half of the EPROM is not addressable
and therefore unused.
The reason for using 2764 EPROMs is probably that by 1992 the older
2732 EPROMs had become obsolete and were difficult to obtain.
The pin-out of the DC37 connector of the B2 cartridge,
is identical to the pin-out of the DC37 connector of the
A1 program cartridge that could be fitted to the
DC37 receptacle at the rear of the GA-082.
In other words: when loaded with appropriate firmware, a B2 cartridge
can be fitted directly to the GA-082. There is one restriction though:
the screws that bind the DC-37 plug to the cartridge, interfere with
the mounting stubs of the DC37 receptacle of the GA-082.
This can be resolved by removing the mounting stubs and replacing
them by regular recessed screws. 1
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The image on the right shows the order and orientation of the two
EPROMs inside a B2 cartridge. EPROM A holds the lower 8-bits
of the data, whilst EPROM B holds the upper 8-bits. The
index notch of both EPROMs must be on the left side, as indicated by
the yellow arrows.
We've tested the above, by burning the USER13 package
into the upper half of the 2764 EPROMs of a B2 cartridge,
and installing it in a GA-082 with firmware version 2.0.
On startup, the display shows:
SYSTEM2.0 USER13 .
The USER13 package does not work with version 3.0 or later.
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The the pinout (and in fact the internal layout) of the
B4 Test Cartridge is identical to the B2 cartridge.
The only difference between the two cartridges is the number printed on the
label. Both cartridge types were supplied with empty EPROMs (or OTPs),
and should be loaded by the user.
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On some GA-082 units this has already been done.
It is also possible to remove the screws at either side of the DC37
connector of the B2 cartridge. This will not make the cartridge weaker.
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© Crypto Museum. Created: Monday 20 January 2025. Last changed: Tuesday, 18 February 2025 - 10:21 CET.
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