TINY-2 repair ideas wanted

[G. Skiffington]

Howdy all - Troubleshooting a problem that developed abruptly with my
TINY-2 TNC (original not the MK2). The symptoms are:

- No display of internal messages including - No LOGON and
- No display of parameters
- No display of received on-air traffic.
- When I have the baud rate between the TNC and the computer correct AND
if I enter "ECHO ON" then I see all commands echoed back to screen BUT
the parameters or results are not displayed.
- Commands DO seem to be interpreted correctly and work normally.
Connects can take place and be disconnected normally. Both CMD and
CONVERSE modes therefore seem ok. On-air monitoring of the databurst
sounds correct and the radio tones and deviation are measured good.

On the surface, it "seems" to me to be a problem with the Z80 SIO chip
not passing data in the TNC to computer direction. I see data on the
CPU/EEPROM/MEMORY databus at pins 1-4 and 37-40 (D0-D7) of the SIO chip
(U14) AND I see good RxDa input at pin 12 (also U14). Looking at the
TINY-2's schematic, it appears that any serial data coming from this SIO
chip should be out of pin 28 RxDB. This data is labelled RXD-2 on the
schematic and shows connected to the MAX231 chip at pin 8 (which concurs
with the MAX231 datasheet for data in that direction). The only time I
ever see data on pin 8 of the MAX231 is when I've commanded ECHO ON (and
I'd assume it's the echoing of the command words (nothing else). The
datasheet info I have for the Z80 SIO seems conflicting, however, in
that
the description for RxDB shows it as an input (not an output).

What am I missing here?
Is the serial output of the SIO actually pin 28 RxDB?
How is off-air data, arriving at pin 12 (U14) sent onward out of the
chip when in converse mode?
What is needed at this SIO chip to cause it to pass the TINY-2's
internal parameter messages to a serial output?
Where are commands "echoed" such that I am seeing the command (verifying
both the baud rate AND the fact that I actually have a path back to the
computer)?

Auxiliary basic info - +5 supply is good
- 3volt battery shows 3 volts
- commands input to assist showing data were:
MCOM ON, MON ON, MCON ON, MALL ON, ECHO ON
- "DATA" when mentioned above, was seen using
a scope, and was good TTL
levels.

Long post....any thoughts appreciated.

Gord - VE1AJF

[G. Skiffington]

Further info: I do see echoed commands/echoed other keyboard typing at
pin 28 of U14......just nothing else coming out of this pin.

Gord