Sometimes, an old machine like the SPARCstation 10 decides to not boot (or at least boot slowly, or boot in such a way that the video output takes a while to show up).
I don't have the correct A/B splitter cable for the serial port, but I do have a PC RS-232 null modem cable, and with three jumper wires, one can improvise a suitable cable to debug boot issues on the SPARCstation (or just watch early boot diagnostics output - for fun!).
Jumpers
The SPARCstation 10 service manual has a description of the jumper settings:
In our case, we want to go for RS-232 serial, to interface with existing PC serial ports (or USB RS-232 serial port adapters).
On real hardware, the jumpers are located below the SBus modules, and you might need to remove the MBus CPU modules as well:
Pinout
The service manual linked above also has this useful pinout of the serial port (the connector actually provides 2 ports, A and B - we're just using the "A" port here):
To get a connection working, it is enough to connect TXD, RXD and GND to the serial terminal (e.g. a PC). In my case, I'm using a null modem cable which takes care of crossing TXD and RXD. Unfortunately, my null modem cable is female, and the connector on the SPARCstation is also female. One option would be to order a gender changer (DB-25 (M) to DB-25 (M)), e.g. Startech has one of those, but if you need it Right Now, that's not an option :)
Fear not, some jumper wires to the rescue - connect pins 2 (TXD aka Xmit Data A) and 3 (RXD aka Rec Data A) as well as pin 7 (GND) directly with the corresponding pins. If you would connect without a null modem cable to a PC serial port, you would need to cross TXD and RXD manually (pin 2 to pin 3, pin 3 to pin 2):
If you are using the default configuration, the settings for
your terminal program (use e.g. minicom or screen on
Linux, Hyper Terminal if you're on some old XP box) are:
9600 8-N-1
Boot up, and you should get super early pre-boot output. You might need to disconnect the keyboard and/or the display to get a working serial console terminal that you can boot from and log in to.
There are some extra resources on SunHELP how to connect the serial port and where to find cables that can be used directly without jumper wires, but this approach worked well for me for the short time I needed to see serial output without having to order extra cables and wait for them to arrive.