LZ: RAM I/O Expander

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LZ: RAM I/O Expander

I was just looking over the plans for the RAM IO Expander and it 
looks pretty good.  The only thing is that it may be slow.  The 
reason for this is that for every byte that the TI-85 wants to read 
or write, it has to transfer four (send 3, get 1 for read and send 4 
for write).  What would greatly speed this up would be to have page 
writes or just have the TI-85 send three config bytes and in them say 
how many bytes to write or read and then just write or read that many 
without the three bytes.  A couple ways to do this would be to add 
the ability to write up to 5 bytes at a time using the two leftover 
bits in the address, or increase the number of data bytes to 4 and 
have the last be the number of bytes.  Either way would work fairly 
well.  It may require a lot more work on the plans, but it would 
greatly speed up the transfer.  Actually, the first way would 
probably work sufficiently and be easier to implement, but the second 
way would speed things up even more, but would require a lot more 
work.  


As an example, if the link port transfers at 9600 bps (BITS per 
second), then it can transfer a maximum of 1200 BYTES per second.  
Now, if you were to use the current plans, which require 4 bytes 
transfered for every 1 data bytes, then a maximum of 300 bytes per 
second can be transferred.  If you were to use the first way 
mentioned, then can transfer 8 bytes for every 5 data bytes.  This 
will speed up the transfer to a max of 750 bytes per second, which is 
150% faster than the current way (2.5 times as fast).  This way is 
definately worth the extra work.  The second way could transfer up to 
256 data bytes for every 4 config bytes.  This would result in 
transfer rates up to about 1181 bytes per second.  This is an 
increase of 294% (3.9 times) over the current plans and a 50% 
increase (1.5 times) over the first method suggested.  If you are 
looking for the maximum transfer rates, then the second way is 
definately the way to go.  And for those of you who are wondering how 
you can get 5 out of two bits, you are always going to send or 
recieve at least 1 byte so just add one to the value that is sent in 
the config byte. (00=1,  01=2, and so on.)


I don't know what you think of this, but I think Mel Tsai would be 
willing to think about one (if not both) of my suggestions.


-Ed
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