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The DECpc LP line of CPU-upgradable machines was announced on 25 August 1992 alongside
the entry level DECpc 333sxLP model with non-upgradable (soldered) CPU. CPU-upgradable
machines started with a 40MHz AMD 386DX with the top model being an Intel 486DX2-66. This
page discusses those CPU-upgradable machines - should I ever get my hands on a 333sxLP I'll
either update this page or document it elsewhere.
The table below shows the prices at launch, along with equivalent 2023 prices accounting for inflation (all in US$):
| Model | List Price (September 1992) | List Price (2023) |
|---|---|---|
| PC736 - 340dx LP | $1,549 | $3,364 |
| PC741 - 425sx LP | $1,649 | $3,581 |
| PC743 - 433dx LP | $1,849 | $4,015 |
| PC744 - 450d2 LP | $1,999 | $4,341 |
| PC746 - 466d2 LP | $2,199 | $4,775 |
These machines feature the CPU on a daughter card allowing easy upgrade from
a 386 to a 486 by just swapping the card. The 486 daughter cards feature a Vacancy Socket allowing
a CPU upgrade by just plugging the new one in and adjusting the jumpers - no need to remove the
old CPU (assuming it can be removed - the 425sx daughter card has the chip soldered, the rest appear
to be socketed). The motherboard itself is the same for both the 386 and 486 models. The white connector
near the unpopulated socket on both cards is for the front LEDs and reset button.
The enclosure includes space for two internal 3.5" hard disks and a regular AT power supply.
On the rear is the usual selection of ports, plus a case lock.
The DECpc LP line originally shipped with MS-DOS 5.0 and Windows 3.1, with later machines almost certainly shipping with MS-DOS 6.0.
There were at least two DOS 5+Windows packages offered by DEC around the time of the DECpc LP lines launch: one from a few months earlier described by SPD 37.30.00 which includes a number of DOS enhancements and drivers for a few prior models (the DECpc 433 Workstation and the DECpc 320sxLP and 325sxLP), and a revised package from September '92 described by SPD 37.30.01 which appears to be less customised.
Most DECpc LP series machines would likely have shipped with the September '92 version, though its possible some early models may have shipped with the earlier package. I don't have a copy of the September '92 set, but the earlier one is included below:
Documentation:
Media:
These machines originally shipped with a Rayovac 840 4.5V alkaline battery
attached to the motherboard next to the power connector with Velcro. This battery will leak eventually
and cause serious damage, so it must be removed on sight. Normally these batteries aren't such a problem as
they're typically attached to some part of the case. But a DECpc they're no better than the infamous Varta:
I've replaced the battery in my working machine with three Energizer Ultimate Lithium cells. These are guaranteed never to leak (time will tell I guess) and supposed to have a long shelf life. These batteries actually have an open circuit voltage of 1.8V putting three of them at around 5.4V - quite a bit higher than the original battery.
In this machine the battery is connected to the VCC pin on the 82C206 Integrated Peripheral Controller through a diode and a resistor when the power is off. The diode is located on the back of the motherboard near the battery connector and has a voltage drop of 0.56V. The battery voltage after the diode should be around 4.8V while the 82C206 VCC max is 6.7V, so I'm satisfied this is safe.
Thanks to the diode, connecting the battery the wrong way round won't damage anything, but it also won't work.
The battery plugs in to J12 with the positive pin being the one nearest the Floppy connector. The pin at the
pin the opposite end is negative.
Like the DECpc 320sxLP and 325sxLP models, the power switch is inside the power supply. The button
on the front of the computer is connected via a linkage to the power supply switch with a short rubber tube
that fits over the end of the linkage and the switch to keep them aligned. On every machine with this setup
I've encountered the tube has disintegrated leaving the end of the linkage floating unaligned with the power
switch. This results in the button on the front of the machine not working.
My solution so far has just been to make a replacement tube by rolling up a small bit of paper and wrapping a
bit of sticky tape around the outside. It's not an ideal solution, but it works ok.
These machines are a bit picky when it comes to memory. They likely only work with FPM SIMMs and the service manual says to make sure to:
| Bank 0 | Bank 1 | Bank 2 | Bank 3 | Total |
|---|---|---|---|---|
| 4 MB | 4 MB | |||
| 4 MB | 4 MB | 8 MB | ||
| 4 MB | 4 MB | 4 MB | 12 MB | |
| 4 MB | 4 MB | 4 MB | 4 MB | 16 MB |
| 4 MB | 16 MB | 20 MB | ||
| 4 MB | 4 MB | 16 MB | 24 MB | |
| 4 MB | 4 MB | 16 MB | 16 MB | 40 MB |
| 16 MB | 16 MB | |||
| 16 MB | 16 MB | 32 MB | ||
| 16 MB | 16 MB | 16 MB | 48 MB | |
| 16 MB | 16 MB | 16 MB | 16 MB | 64 MB |
Is limited. Hard disk types 48 and 49 will let you enter the CHS values yourself but there is no LBA support or auto-detection. I've not tested CF cards with these machines, but the DECpc LPv/LPv+ are known to have issues with them and I doubt this older model is any different. Probably best to use an XT-IDE ROM if you want large drive or CF card support.
These machines have a Turbo LED next to the reset button (pictured right) but no turbo button!
You can turn Turbo on and off via the BIOS setup program. It's over on page 3 labeled CPU Speed.
The slow option should be equivalent to 8MHz:
| Name | Model | RAM | CPU | HDD | CD-ROM | Cards | Condition/notes |
|---|---|---|---|---|---|---|---|
| 340dx LP | 8MB 128K secondary cache |
386DX-40 | — | Digital DE200 network | Working 30/01/2024. Model PC736, built September 1992. PhoenixBIOS A386 Version 1.01.02, DECpc 386dx Version 1.11. |
||
| 433dx LP | — | 486DX-33 | — | — | — | Dead 30/01/2024. Never worked as long as I've owned it (since ~2003). No video output. POST card gives 0A09 with no RAM, or 2725 with RAM. Machine produces a few beeps with a strange low tone (likely memory error - there was none installed for this recording, but the tone is wrong which indicates some other more serious issue). Model PC743, built June 1993. No hard disk bracket. Power button clips broken. |