30 May
2015
30 May
'15
11:29 a.m.
On 05/30/2015 10:40 AM, tony duell wrote:
Is there a good reason why filament lamps were used on
minicomputer
front panels until the mid 1970s? Things like the PDP11/45, Philips P850, etc
all used filament bulbs, not LEDs.
I can think of a few reasons.
First, a filament bulb has the effect of visibly "stretching" a visible
pulse.
Second, a filament lamp in an indicator application with a "keep warm"
provision can have an extremely long lifetime. Nobody knew what to
expect as the lifetime of a GaAsP device.
Third, I'm not entirely certain what the supply situation was in, say,
1974. Incandescent lamps were plentiful and available and inexpensive.
But new equipment certainly had LEDs as indicators in 1973.
Fourth, inertia. Consider how long it's taken for the automotive
industry to adapt to LEDs for even the simplest indicator applications.
Telcos were similarly slow on the adoption of LED technology.
The reliability problem was real--I still have some 7-segment displays
from the mid-70s that have non-working LED segments.
--Chuck