Re: TI-85 as ASCII terminal

To: CALC-TI@LISTS.PPP.TI.COM
Subject: Re: TI-85 as ASCII terminal
From: Tom Esposito <thomase@PRINCETON.EDU>
Date: Sat, 6 Mar 1999 11:49:48 -0500
Organization: Princeton University
References: <7bpnhd$ok3$1@cnn.Princeton.EDU> <7bpq07$j3g$1@ns2.ryerson.ca>
Reply-To: Tom Esposito <thomase@PRINCETON.EDU>
Xref: paladin.american.edu bit.listserv.calc-ti:28335

I dug up some stuff on the TI-85 protocol and it looks something like this:
All ASM code refers to two lines R and W which I can only guess stand for
Red and White (wire color).  By activating the line you pull it low and by
deactivating it you leave it high.  I guess they are open collector outputs.
On the TI, you can read port 7 and bits 1 and 0 hold the values of R and W
respectively.  Now here is where it gets weird: you must write these
seemingly randomly selected command codes to port 7 in order to turn the
output on or off:
    R=0 W=0    write 0xFC
    R=0 W=1    write 0xD4
    R=1 W=0    write 0xE8
    R=1 W=1    write 0xC0
First of all, to use the protocol it seems to me that you have to agree on
who is sending and who is receiving.  So, sender pulls R low to send a 0 and
pulls W low to send a 1 and then continues to poll the port.  When the
receiver has recieved the bit, it pulls the opposite line low.  The sender
sees this and then lets go of the line it pulled.  The reciever lets go of
its line.  Repeat the process.
No wonder why the TI-Graph link cable uses a PIC microcontroller.  To
directly convert this to and from RS-232 takes a UART to receive the serial
and a micro to implement the TI protcol.  Does this seem right?

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