[Ham-Mac] Need help with PK-88 TNC

Sun, 4 Apr 2004 07:50:40 -0700 (PDT)

Hi Again David and All,

I really appreciate the responses I have gotten. Pin
13 gets its input through a 100 ohm resistor from the
RS-232 port on the PK-88, pin 2 labeled TXD on the
PK-88 schematic. I have checked the signal from the
Mac with a logic probe and it makes it through all the
way to pin 13, but there is no output from pin 11
which should go into pin 27 (labeled RXDB) of IC-12,
which is an 8530A. Pin 12 on IC-4, the 75189, is the
threshold control for that particular line receiver,
and I believe that there are a total of 4 in the chip
of which only 2 are used as far as I can tell.

I think you may be on to something there David, and
also another poster suggested that the output from the
Mac's serial port is below RS-232 levels, so that may
be another issue also. I am going to check the value
of the input resistor to pin 13 and make sure it isn't
out of tolerance, maybe attenuating the signal enough
to keep it from passing all the way through. 

Thanks again for your continued assistance,
Carl KA5KAB

--- David Anderson GM4JJJ <[email protected]> wrote:
> Hi Carl,
> 		I don't have a PK-88 manual here anymore, so
> Unfortunately I can't 
> check the circuit diagram.
> PIN 12 is associated with the input line going to
> PIN 13. Can you check 
> where PIN 13 on the IC4 goes to, I imagine one of
> the DB-25 pins.
> 
> This will tell us whether it is the Receive data
> line or possibly a 
> hardware handshake line.
> 
> It may be that your modem cable is missing a line
> going to that input 
> and by putting a probe on teh threshold input you
> are simulating the 
> effect of having an input on that pin 13 by changing
> the threshold at 
> which pin 13 changes state.
> 
> 73
> 
> David
> On 4 Apr 2004, at 1:56 pm, Carl Miller wrote:
> 
> > Hi David and All,
> >
> > I am using a DB-25 to the Apple 8 pin connector to
> > connect the tnc to the Mac. I believe it's a
> standard
> > Apple modem cable and I have it connected to the
> modem
> > port.
> >
> > My terminal program is Microphone. It came with a
> > Supra modem I bought and I noticed it mentioned in
> the
> > PK-88 manual as one of the programs tested with
> the
> > tnc. It works great as long as I'm touching pin 12
> of
> > IC-4 with a probe of some kind.
> >
> > My settings on the terminal program are:
> >
> > 1200 bauds
> > 7 bits/character
> > even parity
> > handshake- xon/xoff
> > modem connection- telephone port
> > 1 stop bit
> > the only function I can't set according to the
> manual
> > is "full duplex". I don't see that setting in any
> of
> > the Microphone menus.
> >
> > By the way, and I forgot to mention, when I first
> > discovered this unusual procedure I thought
> perhaps it
> > was a bad IC socket so I removed IC-4 from it's
> > socket, removed the socket, and soldered the chip
> > directly to the PCB. It didn't help.
> >
> > If you need any further info just let me know.
> This
> > thing is really got me confused.
> > Thanks again,
> > Carl
> >
> >
> > --- David Anderson GM4JJJ <[email protected]>
> wrote:
> >> Carl,
> >> 		Can you tell us the details of how your serial
> >> lead is wired? Also
> >> what software, is it a regular terminal program
> like
> >> ZTerm or what?
> >>
> >> 73
> >> David
> >>
> >>
> >>
> >> On 4 Apr 2004, at 1:15 am, Carl Miller wrote:
> >>
> >>> Hello All,
> >>>
> >>> I am new to the list and I already need help
> with
> >> a
> >>> problem.
> >>>
> >>> I have a PK-88 TNC that I am trying to use with
> my
> >> old
> >>> Macintosh Classic II.
> >>>
> >>> The problem I am having is that the PK-88 will
> not
> >>> accept commands from the terminal. That is,
> until
> >> I
> >>> touch a probe or other metallic object to pin 12
> >> of
> >>> IC-4, a 75189 quad line receiver. Once I power
> the
> >>> thing up and touch a metallic probe to pin 12
> the
> >>> thing works great, I can send it commands, I can
> >> do
> >>> the loopback test with no problem. But once I
> >> remove
> >>> my probe, the thing goes deaf again. I also
> >> noticed
> >>> that if I remove the probe, then touch pin 12
> >> again it
> >>> gives me the cmd: prompt. Then I can communicate
> >> with
> >>> the tnc again.
> >>> According to the Texas Instruments data sheet on
> >> this
> >>> particular chip, pin 12 is the threshold control
> >> pin.
> >>> But, since I am not an EE, I don't have a clue
> >> what
> >>> that means. The funny thing about pin 12 is that
> >> it
> >>> isn't connected to anything from what I can
> tell.
> >> It
> >>> certainly isn't noted on the schematic.
> >>>
> >>> What gives here? Is this thing posessed or what?
> >>>
> >>> I would certainly appreciate any help or advice.
> >>> Thanks in advance,
> >>> Carl KA5KAB
> >>>
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