[COFF] DEC terminal line driver chips?

[Derek Fawcus]

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