i8355 from SDK-85 flakey

allison ajp166 at verizon.net
Tue Apr 27 17:18:19 CDT 2010


Tony Duell wrote:
>>>> 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.
>
>   
Yes, but the SDK85 design you could jumper the IO for
RS232, 20ma or TTL.

>>> 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. 
>   

It would or a common 9V wall wart.



>> 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 have that too.  The inverter can be jumpered out.

Used a lot of SDK85s over the years with terminals
and it was never an issue.  Wish I'd kept one but I do
have a home brew(wire wrapped) copy of one.

> 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.
>
>   
Again, there are jumper options for that.

Allison
> 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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