AMSTRAD PC3386SX
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(pictures copyright by M.A.Grundke)
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AMSTRAD PLC MODEL:PC3386SX
SERIAL NO 04200316-C8-310411/EU-G
MADE IN U.K. |
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Keyboard serial number:
548-0852636 |
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| PC3386SX keyboard closeup |
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PC3386SX overview:
board details are mentioned below, so let's have a look at some of the PC3386SX's specials. Interesting is the CPU board that AMSTRAD decided to use (lower left, above SIMM bank). It allowed the board to use different CPU-configurations (80286, 80386SX). However, I don't know in how much PCs AMSTRAD used this board and its modularity.
The next interesting thing is the machine's ISA-slot daughterboard in the leftmost 16 bit ISA slot (middle, left from floppy cables etc). So you've got 2x 8 bit and 3x 16 bit slots in that compact desktop case - not bad. Then, there's the machine's drive bays: 2x 3.5", 2x 5.25" drives. These drives can be easily slided in and out - there are plastic holders mounted to them, fitting into the appropriate splints in the case. Clever! Note that CPU- and ISA daughterboard are, in detail, discussed below |
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PC3386SX board overview:
beginnig left, there's the front mouse port, which can only take AMSTRAD-mice (not a serial port!). Right from there, the RAM bank (30 pin SIMMs). Behind both you can spot the five 20 pin connectors used for the CPU daughterboard. Right from these, the square AMSTRAD 40230 (IC120), and, again right, the Zilog FDC (IC172, with three oszillators). Below the FDC, floppy- and HD-connectors, and still below the SED9420C data separator (IC801).
Getting back to the CPU board connectors, there's 11 pcs. 16Kx4 SRAM (IC160-IC170, 64K cache+parity) above them, and still above, the 4 pcs. 32Kx8 AMSTRAD ROM BIOS (IC149-IC152). Right from there, the square AMSTRAD 40231 (IC134), and the three onboard ISA-slots (with daughterboard power connector below leftmost). Still right, the Paradise PVGA1A-JK (IC307), with 8 pcs. 32Kx8 SRAM (video RAM, locations see below) above. Above are the board's power connectors and the inmos IMSG171P RAMDAC (IC305) with VGA connector and VGA DIP-switches behind. Finally, left from the slots, are a Toshiba T4758A (IC115, beneath serial & parallel ports), a SiS 82c450 UART (further left, IC110)
and a DALLAS DS1287 RTC (IC116). Behind the RTC, the DIN keyboard connector (AMSTRAD-compatible keyboards only). By the way, there's lots of programmable logic (PLAs) on both sides (!) of the board |
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The machine's ISA slot daughterboard, allowing you to use three more (vertically mounted) expansion boards, at the cost of one onboard-slot (where the daughterboard plugs in). Right from the 16 bit ISA-connector you can see a 4 pin power connector, which plugs into the mainboard to provide expansions on the daughter with the necessary voltage levels. As you can see, the board is not completely fitted - there are soldering pads for another 16 bit slot and for upgrading the 8 bit slot to 16 bit. Most
probably they were left out for cost-reduction and can be upgraded if required. There's also silkscreening on top of the board (and on its back):
ISSUE B 250290
AMSTRAD Z70923 DPA1706 [ ]
MADE IN THE U.K. |
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board silkscreening, part 1 (below CPU-daughterboard):
DPA1704
[ ]
AMSTRAD Z70921
MADE IN THE U.K.
To the left and right you can see part of the five daughterboard-connectors |
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board silkscreening, part 2 (both sides of mainboard):
ISSUE C
030490 |
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| The PC3386SX's CPU, an Intel NG80386SX-20 (in C-STEP), running at 20 MHz. This CPU was Intel's counterstrike against the masses of 286-clones these days; the 80386SX was a 80386 stripped down to a 24 bit address-bus so that it would fit into existing and cheap 286 board-designs. 'NG' describes the package used: the NG80386SX comes in the so-called 100-lead plastic quad flat package (PQB-100). To the left and right, by the way, you can see two of the 20 pin CPU daughterboard-to-mainboard connectors |
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Overview of the CPU daughterboard, with the NG80386SX-20 on the left side. As mentioned ealier, using a CPU daughterboard is a clever solution for building versatile mainboards. AMSTRAD could use the same mainboard with different CPU-board in different models, e.g. fitting it with 286 or 386SX-CPUs (maybe even others, but this would result in more logic on the CPU-board, since other CPUs have different data- and address-buses). Besides the CPU, there's also an FPU-socket there (to take a 80387 FPU). There's a jumper (LK720, 'LK' standing for 'link') on top of the
board - I assume for FPU/ no FPU operation. And there are four F245 bus buffers and a TI 16L8, for handling the CPU-to-mainboard bus (necessary because usage of a daughterboard increases signal latencies etc). Finally, there's also a silkscreening on both sides of the CPU-board:
(C) AMSTRAD PLC 1990
2600-025P-2
RB-2
SB-2 CB-1 |
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| These are 11 pcs. Toshiba TC55417P-35 (16Kx4, 5V SRAM). I assume that 64 KBytes (8 pcs. 16Kx4) are used as CPU-cache, the rest (3 pcs. 16Kx4) are used for generating parity bits. That's a little odd, since 2 pcs. (=8 bit, providing the 9th parity bit to eight SRAMs) should be enough for that... Perhaps a 4 bit tag-RAM? But then it should be faster than the others, but they're all 35ns... An interesting fact is, by the way, that this CPU cache was only used in early machines - later ones came without (who knows why). Well, there are also a lot of jumpers there (what AMSTRAD calls 'option links'). I've only found information on one of them: |
| Jumper | Function |
| LK38 | unknown (1-2 closed) |
| LK41 | unknown (1-2 closed) |
| LK43 | unknown (1-2 closed) |
| LK47 | unknown (1-2 closed) |
| LK26 | unknown (2-3 closed) |
| LK25 | unknown (2-3 closed) |
| LK36 | enable/ disable expansion bus cache |
| LK37 |
unknown (but closed) |
| LK42 | unknown (but closed) |
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| The machine's RAM bank, consisting of 4 pcs. Samsung KMM 59256AN-10, each holding 2 pcs. Samsung KM44C256AJ-18 (256Kx4 SRAM) plus one KM41C256J-10 (256Kx1) for parity. Makes a total of 1 MByte RAM, divided into 640 KB base-memory and 384 KB extended RAM. Most of the PC3386SX were, by the way, reported to have come with 4 MB RAM. Again, lots of 'option links' here (from left to right: |
| Jumper | Function |
| LK27 | enable/ disable onboard HD-controller |
| LK34 | enable/ disable POST (power-on self test) |
| LK33 | enable/ disable serial port |
| LK32 | enable/ disable onboard FDC-controller |
| LK31 | enable/ disable parallel port |
| LK28 | enable/ disable onboard VGA-controller |
| LK29 | unknown (but closed) |
| LK30 | enable/ disable mouse port |
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| The machine's 128K ROM BIOS, stored in 4 pcs. ST27256-20CP (32Kx8 EPROMs). This BIOS seems to have been developed by AMSTRAD. The BIOS-message is 'AMSTRAD PC3386SX (v2.1) (c)1988,1989,1990 AMSTRAD plc'. Later AMSTRAD-PCs, however, came with a BIOS developed by Quadtel (a Phoenix-subsidiary). But maybe this AMSTRAD-BIOS is a Phoenix-OEM? However, there are also two jumpers: |
| Jumper | Function |
| LK13 | unknown (but closed) |
| LK14 | unknown (but closed) |
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| A common part is the DALLAS DS1287 RTC (RTC=real-time clock). It is pin-compatible with the Motorola MC146818A and can store its parameters for up to ten years (being powered by a lithium cell). As the name implies, the DS1287 stores date and time, and continues counting once they have been set (always providing the machine with 'real-time'). It is a very flexible RTC that can be used in both Intel- and Motorola-designs. Also interesting is that it comes in a 'encapsulated package' - most probably because of safety issues (lithium cell inside). To the left, however, part of the machine's keyboard port |
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| The PC3386SX's onboard VGA-controller, a Paradise PVGA1A-JK (made by Western Digital). It can display the standard VGA-modes (i.e. 320x200x8, 640x480x4, 640x480x8) plus some 'extended modes' such as 640x400 (256 colors), 800x600 (monochrome or 16 colors). These extended modes can be disabled for compatibility, by setting some of the PVGA's registers (which is also done by setting DIP switch 2, see below). However, the Paradise PVGA was a cheap and easy-to-implement VGA-controller and thus very popular these days. It was succeeded by WD90C00, WD90C10 and WD90C11-series controllers |
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| The inmos IMSG171P-35G is the machine's VGA RAMDAC. Like all RAMDACs, it is used to convert the digital data produced by the VGA controller into analogue data (DAC=digital-to-analogue converter), i.e. analogue signals for red, green and blue. These analogue signals can then drive the attached display. The data to be converted is fetched from the machine's video RAM into the RAMDAC's so-called 'color lookup table' (a little amount of buffer RAM in the RAMDAC) before the conversion. Members of the inmos 171-family were, by the way, also used in IBM's PS/2-series (which introduced the VGA- and PS/2-standards to desktop-PCs). Below the RAMDAC, you can see a LM339N, a LM139-series voltage comparator often found in DAC-circuitry |
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| These 8 pcs. Texas Instruments TMS4464-12NL (32Kx8 SRAM) are the machine's video RAM, 256 KBytes. Not much, but very common these days and enough for the most important VGA display modes (such as 320x200/ 640x480 in 256 colors) - there were even some 'bonus' resolutions, like 800x600 in 16 colors. But even with more video RAM, the PVGA1A couldn't do more |
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| The machine's onboard floppy disc controller, a Zilog Z0765A08PSC FDC. It's a NEC µPD765-compatible FDC, which was also used in many other PC-designs (although it's quite special for an AT-class machine like the PC3386SX - ATs mostly utilize the Intel 82072). Its function is to provide basic diskdrive control (such as starting and stopping the drive motor, set the device busy etc). The FDC can also initiate data transfer to or from the drive. By the way, this FDC has a workaround - one of the pins is connected to the board with two resistors. Don't know what it's for, and if this was done in the whole series, or just the early machines... |
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| This SED 9420Cac is a data separator which was also used in other AMSTRAD's (like the PCW8256/ PCW8512). Don't exactly know what it does, but (guessing from the name) I think it transfers the data gathered by the FDC into RAM (although the FDC is DMA-capable itself - hmm) |
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| The SiS 82C450 is a CMOS-version of Intel's 82450 UART (UART=universal asynchronous receiver & transmitter). The UART drives the serial line drivers & receivers, thus controlling the machine's serial port. The 82450 is the 16 bit successor to the 8250 UART (which was 8 bit and used in XT-class machines), and should be compatible to the 16450 UART. Since it's got no FIFO buffer ('first-in, first-out'), like the later 16550, transfer rates higher than 14400 bits/s can cause trouble (e.g. when using fast modems) |
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| The machine's serial quad-line driver (DS1488M, left picture) and quad-line receivers (2x DS1489AM). These are used to produce the output levels needed for serial (i.e. RS232-compliant) communication. They're controlled directly by the 82C450 UART, and do the 'low-level serial communication' (while the UART handles the higher protocols, and doesn't have to care about the signal levels required) |
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| An unknown chip, the AMSTRAD 40230 at IC153. It's located between CPU-daughterboard, RAM bank and FDC, so perhaps it is AMSTRAD's implementation of a DMA controller? Above it, however, is a jumper block: |
| Jumper | Function |
| LK17 | unknown |
| LK13 | unknown (but closed) |
| RAM size (SIMMs used) | LK19 | LK20 | LK21 | LK22 |
| 1024KB (4 pcs. 256KB SIMM) | open | open | closed |
closed |
| 4096KB (4 pcs. 1024KB SIMM) | closed | open | open | closed |
| 16384KB (4 pcs. 4096KB SIMM) | closed | closed | closed |
closed |
| RAM speed | LK23 | LK24 |
| 80ns | closed | closed |
| 100ns | open | closed |
| 120ns | closed | open |
| 150ns | open | open |
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| Next unknown chip: AMSTRAD 40231. Resides between ROMs, ISA-slots and I/O-area. A bus-controller, perhaps? |
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| Toshiba T4758A, a LSIC (large-scale integrated circuit). Again, this chip is unknown and I didn't find any document describing it. It's located in the machine's I/O-area (before the ports, next to UART and RTC). Perhaps it's an interrupt-controller (since the PC3386SX doesn't seem to have a PIC)? Behind it, however, some jumpers that control the AMSTRAD-specific front mouse port (from left to right): |
| Mouse port IRQ | LK9 | LK10 | LK11 | LK12 |
| IRQ 3 | open | open | open | closed |
| IRQ 4 | open | closed | open | open |
| IRQ 5 | closed | open | open | open |
| IRQ 9 |
open | open | closed | open |
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| Another unknown chip: Toshiba T9035H. Whatever it does, I don't know (perhaps it's just TTL?) |
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| Also unknown: DS1013M-15 and D51000M-100. Before them, three jumpers (from left to right): |
| Jumper | Function |
| LK3 | unknown |
| LK1 | unknown |
| LK2 | unknown (but closed) |
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| The PC3386SX's onboard VGA connector (standard 15 pin SUB-D). To the right (and that's what's more interesting) DIP switches, controlling the machine's video output: |
| DIP-switch | Function |
| SW1 | ON: Multi-frequency monitor attached
OFF: fixed-frequency (31.5kHz only?) |
| SW2 |
ON: 'PS/2 mode'
OFF: 'AT-mode' (= PVGA extensions enabled?) |
| SW3 | unknown (unused?) |
| SW4 | ON: 16 bit VGA bus interface
OFF: 8 bit VGA bus interface |