i8355 from SDK-85 flakey

Tony Duell ard at p850ug1.demon.co.uk
Tue Apr 27 14:43:32 CDT 2010


> >> The tty interface on this SDK-85 was built for 20mA current loop so I'll
> >> have to either rework that to RS232 or invent a new method to offload.
> >> But not a big deal really.
> >>     
> >
> > I would be _very_ supprised if that serial interface doesn't sue some 
> > TTL-level serial chip (or bit-banged through a TTL level port) followed 
> > by a current loop interface. 
> >
> >   
> It does not.  The DSK85 does bit bang through the SID and SOD pins
> on the 8085 CPU.

I would argue that is 'bit banged throug ha TTL level port'. The fact 
that that port happens to be on the processor chip doesn't alter the fact 
that tehre's a TTL level serial signal you can use.

> 
> > In which case, since you need output only, grab the TTL leve signal at 
> > the input to the coverter and feed it into something like a MAX232 (or a 
> > 1488 if you have +/-12V suppleis around). 
> >
> >   
> No need.  The SDK can be via jumpers wired with run RS232 levels
> for terminal. A minus 5 to 12 Volt supply is required but we are
> talking a few milliamps.

If all else fails I suspect a common 'PP3' (as we used to call them) 9V 
battery would do the trick. 

> 
> The intel i8085 manuals had the schematic for the SDK85.

So does the book I've just pulled off my bookshelf ('Microcomputer 
Theory and Applications wit hteh Intel SDK-85'). I can't find any links 
to set RS232 levels on the serial port. The current loop converter is a 
simple trasistor stage (PNP transsitor), emiter to +5V, collector to the 
current loop through a resistor. Now, IIRC, the current loop passes 
current in the idle state (whcih implies that transistor is turned on in 
the idle state), but the RS232 signal is -ve in the idel state. So you 
need an inverter. 

I wonder (and I've not got an SDK85 to try it with). If you connected a 
suitalbe pull-down resistor (say 1k) between the current loop +ve output 
(pin 13 of J7 on my schematic) and ground, and then also connected pin 13 
of J7 to the input of an RS232 driver, would that work? I think so.

Incidnetlaly, the book also has the monitor source lisitng in it. I don't 
feel like typing it in, though :-)

-tony



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