30 Apr
2021
30 Apr
'21
10:49 p.m.
On Thu, Apr 29, 2021 at 09:17:27AM -0400, Clem Cole wrote:
If you look at the schematics for the VT-100 (which
are available on
bitsavers) as well as most terminals of the day used the
Motorola MC1488/1489 driver pairs [early on, TI marked them as 75488/75498
but I doubt you'll see that old].
Anyway, for any terminal of those times, I'd be surprised if the later
devices (like VTx20 series) did not them [the issue with these devices is
you need all of 5 and +/- 12 volts]. These terminals are new enough that
they might have used MAX232 (single 5 v power - there are even 3.3v
versions these days). All of these devices are very available online. I
would suggest putting high quality (*i.e.* auget style / machined pins)
sockets. The nice thing about 1488/1489 is that they were doped to fuse
the output section on failure, so just replacing them will in fact
resurrect most serial port failures. Frankly, I never understood why more
manufacturers did not put them in sockets for easy replacement.
Well even in 1988 when I entered industry for a year (as an EE), the MAX232
was quite new (looks ike it was introduced in '87), and plenty of designs
still used the 1488/1489 chips. I do recall that those I worked with at the
time were often produced with turned-pin sockets, specifically to allow the
chips to be easily replaced.
It seemed to be around 91-92 before the MAX232 was common in new designs,
certainly I used it for adhoc test equipment around that period.
I guess familarity, and the the time for the design pipeline to flush out
may have had a effect, only using it on new designs, or reworks where the
PCBs had to be redesigned due to chips going out of production.
DF