ASCII art Katakana - no kanji codepage required! As noted by microlith, this is not romaji but a phonetic approximation.
---. / -+-. --- --+-
|/ /| / | /|
| | / --- / |
ah ee ü eh o
-+-. -+- /--. /--- ---
| | -+- / | |
| / | / / ---
kah kee kü keh ko
-++- - / --. -+-. | /
|| -/ / | /
/ / /\ '- /
sah she sü seh so
/--. -+- ||/ --- |
./ -+- / -+- |\
/` / / / | \
tah tchee tsü teh tow
| --- ---. -- /
-+- ./ ---. /
/ --- /` /|\ /
nah nee nü neh no
/ \ |-- --- -+-
/ \ | / /\ /|\
'-- / \
ha hee fü heh ho
---. --- / \/ -+-
\/ --- / /\ -+-
\ --- /--| / +-
ma me mü meh mow
-+-. --. --+
| | --+
--+- --+
yah yü yo
--- | | | | | +-+
---. | | | | | |
/ / / |/ |/ +-+
rah ree rü reh row
lah lee lü leh low
.--. --+
| --+
/ /
wah woe
- /
/
/
n'
-+-." -+-" /--." /---" ---"
| | -+- / | |
| / | / / ---
gah ghee gü geh go
-++-" - /" --." -+-." | /"
|| -/ / | /
/ / /\ '- /
zah gee zü zeh zoh
/--." -+- " /" ---" | "
./ -+- / -+- |\
/` / / / | \
dah dee djü dzeh doe
/ \" |--" ---" " -+-"
/ \ | / /\ /|\
'-- / \
bah bee bü beh boe
vah vee vü veh voe
/ \o |--o ---o o -+-o
/ \ | / /\ /|\
'-- / \
pah pee pü peh poe
When reading manga, the translators will sometimes leave in katakana in for japanese onomatopoeia. It's handy to have a printout of the katakanas to back-translate those ka-shin (blade strikes) and bo-ro-mu (explosions), instead of having to guess all the time.