As you may have already determined from all the responses not directly directing you to a protocol, that a roll your own approach to be your best choice.
So, this got me thinking and well, here are a few of my thoughts --
Given that this chip has 6 ADC channels, most likely you are using Rs-232 serial comm (a guess from your question), and of course the limited code space, defining a simple command structure will help, as Adam points out -- You may wish to keep the input processing to a minimum at the chip, so binary sounds attractive but the trade off is in ease of development AND servicing (you may have to trouble shoot a dead input 6 months from now) -- hyperterminal is a powerful debug tool -- so, that got me thinking of how to implement a simple command structure with good reliability.
A few general considerations --
keep commands the same size -- makes decoding easier.
Framing the commands and optional check sum, as Adam points out can be easily wrapped around your commands. (with small commands, a simple XOR/ADD checksum is quick and painless)
I would recommend a start up announcement to the host with the firmware version at reset - e.g., "HELLO; Firmware Version 1.00z" -- would tell the host that the target just started and what's running.
If you are primarily monitoring, you may wish to consider a "free run" mode where the target would simply cycle through the analog and digital readings -- of course, this doesn't have to be continuous, it can be spaced at 1, 5, 10 seconds, or just on command. Your micro is always listening so sending an updated value is an independent task.
Terminating each output line with a CR (or other character) makes synchronization at the host straight forward.
for example your micro could simply output the strings;
V0=3.20
V1=3.21
V2= ...
D1=0
D2=1
D3=...
and then start over --
Also, commands could be really simple --
? - Read all values -- there's not that many of them, so get them all.
X=12.34 - To set a value, the first byte is the port, then the voltage and I would recommend keeping the "=" and the "." as framing to ensure a valid packet if you forgo the checksum.
Another possibility, if your outputs are within a set range, you could prescale them. For example, if the output doesn't have to be exact, you could send something like
5=0
6=9
2=5
which would set port 5 off, port 6 to full on, and port 2 to half value -- With this approach, ascii and binary data are just about on the same footing in regards to computing/decoding resources at the micro. Or for more precision, make the output 2 bytes, e.g., 2=54 -- OR, add an xref table and the values don't even have to be linear where the data byte is an index into a look-up table ...
As I like to say; simple is usually better, unless it's not.
Hope this helps a bit.
Had another thought while re-reading; adding a "*" command could request the data wrapped with html tags and now your host app could simply redirect the output from your micro to a browser and wala, browser ready --
:)