7 Nov
2011
7 Nov
'11
5:03 p.m.
On Nov 7, 2011, at 6:48 PM, Glen Slick wrote:
On Mon, Nov 7, 2011 at 3:32 PM, <cctech at
vax-11.org> wrote:
It's all GPIO coming from a parallel register (addressed by one of the decoding PALs)
and then bodged up with a series of diodes and caps to vaguely resemble RS-232. It's
bit-banged in the 8086 (I've disassembled that part, at least). I don't get any
activity whatsoever on the port, though, which leads me to believe something is wrong
elsewhere in the logic. Looking at the input line, at least, on my scope shows me that
the RS-232 line is getting through (at proper logic levels) at least to the GPIO latch
where it's supposed to be.
One of these days it might be useful to figure out something resembling an equivalent
schematic, but since it appears to be a 4-layer board (with lots of lines going under
soldered-down chips besides), it doesn't look like anything I'll be doing soon.
I have some new 53C90A parts on order from recycledgoods.com as well as some 53CF94s for
some personal projects. If anyone is looking for replacement ones, they have an OK deal
on them now (though they gouge on shipping, acting like each chip is a pound).
- Dave
One particular TTL <--> RS232 chip (I forget the PN) had the really annoying
problem of blowing up if it was connected to a powered-up terminal while the
chip was powered down. This may be the cause of your dead UART. Beyond that
I don't have any suggestions.
Clint
It's not clear to me what is used for the RS-232 interface on the
CQD-220. I don't see any obvioius RS-232 interface chips. There are
a handful of diodes and resistors adjacent to the RS-232 port. Maybe
those make up the RS-232 interface. There is no UART on the board. I
have never been curious enough to try to trace the signals to try to
determine how the serial interface is implemented.