I've made the Keyspan USB -> Mac serial adapter in conjunction with ZTerm work for me. I use this to connect to router and switch console ports. The Keyspan USB PDA should work as well, but the trick to either adapter is having the right cable to interconnect your device.
If you go with the Keyspan 2 port USB -> Mac serial, here are the Mac serial pin outs and descriptions to help you with a cable choice.
Note: Ground pin 8 for standard RS232. Use Tx/Rx + and - for balanced serial connections. I believe that would be RS422.
8 pin mini DIN Mac serial pinout:
1) HSKo output handshake (DTR)
2) HSKi input handshake (CTS)
3) TxD- (transmit data for RS232)
4) Ground
5) RxD- (receive data for RS232)
6) TxD+
7) Not connected
8) RxD+ (ground this to emulate RS232)
Here is a pinout for a female DB9 Null Modem adapter. You can use this pinout to see how the signals are matched between DTE (Data Terminal Equipment) and DCE (Data Communications Equipment). You should be able to match signals for your Sun Station's serial DB25. The DB25 serial pin outs I don't have handy but should be easily found on the net.
Null Modem Adapter Pinout:
(DB9F_DTE) (RJ45F_DCE)
(DCD) 1 -
(RCV) 2 ---- 6 (XMT)
(XMT) 3 ---- 3 (RCV)
(DTR) 4 ---- 2 (DSR)
(GND) 5 ---- 4 (GND)
(DSR) 6 ---- 7 (DTR)
(RTS) 7 ---- 1 (CTS)
(CTS) 8 ---- 8 (RTS)
(RNG) 9 -
(DB9F_DCE) (RJ45F_DTE)
(DCD) 1 ---- -
(XMT) 2 ---- 6 (RCV)
(RCV) 3 ---- 3 (XMT)
(DSR) 4 ---- 2 (DTR)
(GND) 5 ---- 4 (GND)
(DTR) 6 ---- 7 (DSR)
(CTS) 7 ---- 1 (RTS)
(RTS) 8 ---- 8 (CTS)
(RNG) 9 -
