[Ham-Mac] Need help configuring MacTNC

[Lew Phelps K6LMP]

Hey, I could use some advice from anyone out there who is running a  
PK-88 packet modem with MacTNC.

I bought a used PK-88 on eBay recently.  It appears to be functioning  
normally, but I am unable to get it to run its basic baud-checking  
sequence.

I am running MacTNC 1.2  The PK-88 is connected through a rewired 25- 
pin to 9-pin modem cable, with the following lines connected:
TXD to TXD
RXD to RXD
GND to GND

The serial modem cable is connected to a Keyspan 19USA serial-USB  
converter. The pin-outs used for the modified cable match up with the  
pins on the USA10.

When I power up PK-88, I get the expected "Type a star (*) to  
begin...." message, but typing an asterisk followed by CMD-T  
("transmit") produces no action with the TNC.

MacTNC settings:  1200 baud, 7 bit, even parity, one stop. XON turned  
on.  RTS and DTR turned off (not checked).

If anyone is using this combination of hardware and software, any  
advice would be appreciated.

Also, are there any other applications I should be considering, other  
than MacTNC, to control the PK-88?

Thanks,

Lew K6LMP