Re: TI-H: RS232 emulation on the link port

[Prev][Next][Index][Thread]

Re: TI-H: RS232 emulation on the link port

On Mon, 18 Nov 1996 17:41:36 +0000, you wrote:
>Everyone discusses the possibility of using a Graph Link to hook a modem
>to a TI and a program such as Talk-85 or (the currently nonexistant?)
>ZTerm. I know this has been discussed before, but I think it should be
>possible to emulate the TxD and RxD lines with the link port using just
>software. It won't be easy, but maybe you could use the self-counting R
>register, which is normally used as a memory refresh counter in the Z80
>processor, as a sort of timer?
>
>I've calculated the clock frequency of my (unturboed) TI to be about 6,3
>MHz with a ZShell program. I estimate it would be possible to make the
>link port work as fast as 9600 bps with only software on an unturboed
>calc, maybe even more (14400, 19200 bps) with skillful programming.
>After the IR link is completed (I cooperate with Keith Burzinski to make
>it) I might start making a RS232C emulator terminal program.
>
>I need some information though: Does anybody know how long a M1-cycle is
>compared to a clock (T) cycle? Some books say it's 4 T-cycles, but the
>datasheets show it can't be exactly that. Maybe around 3,5?
>Actually, the question is how many M1-cycles an unturboed TI-85 executes
>per second. Does anyone know?
>


An M cycle ("machine cycle") varies quite a bit from instruction to
instruction.  It would be easier to just igmore the number of M cycles
and just use T states, because those are always constant (on a TI-85
it's about 167 nanoseconds, but since it doesn't use a crystal it's
more difficult to time).  It should be possible to emulate Rs232, but
it must be "tuneable" because different calculators will have slightly
different operating frequencies, and since they probably don't use NPO
stable capacitors, it could vary by as much as 20% going from 50
degrees F to 90 degrees, enough to screw up transmissions.  Capacitors
and resistors also "age", so older calculators will also have slightly
different frequencies.  Software emulation will be very difficult due
to the lack of an accurate clock, but it's possible if you know what
you're doing.


-Mel


<pre>
--
The TI Memory Expansion Homepage
http://pilot.msu.edu/user/tsaimelv/expander.htm
</pre>
References: