You will find it impossible to do any work with your TS-7800 unless you have a serial console connection. You will need a NULL modem wired serial console cable and a suitable communications program. If you are using Linux or some flavour of UNIX then you will probably want to use minicom, if you are completely nuts and trying to do Linux embedded development using Microsoft Windows then you should use PuTTY.
Once you have everything hooked up, you should configure the connection as such:
| speed: | 115200 |
|---|---|
| bits: | 8 |
| parity bits: | no |
| stop bits: | 1 |
| flow control: | off (both sw and hw) |
If correctly configured, when you fire up your TS-7800 you should see the boot messages from the TS-BOOTROM and then Linux loading its-self before dropping you onto a root command prompt.