i8355 from SDK-85 flakey

Tony Duell ard at p850ug1.demon.co.uk
Wed Apr 28 13:46:20 CDT 2010

> And Tony, you are correct on the serial interface.  It is sourced by the
> SOD pin of the 8085 and then a single transistor drives the current loop.

It was Allison who said it used the SID and SOD pins of the 8085. However 
this agrees wit hthe schematic in the book I have.

I can't find the links that Allison mentioned to seleect TTL/current 
loop/RS232 mode.  So either there are 2 versions of the board or my 
scheamtics are 'simplified'. The only lionk I can see selects whether SID 
comes from the current loop interface or is held low. If I am reading the 
monitor lisitng correctly, then the bit of code starting at 0x0049 checks 
to see if the SID pin is high or low. If the former it assumes a working 
current loop device is connected (idle state would be to have the Tx loop 
complete. so Q1 would be turned on, so SID high) and runs the teletype 
monitor, if the latter, it runs the keypad/7-seg display monitor.

It's possible I've totally mis-uderstood this, I am not really a 
programmer, and I've only just started looking at the SDK-85 monitor 
source listings.

> You need to invert this before feeding into a 1488 or equiv RS232
> driver but that's no big deal.   All the TTY I/O runs at 110 baud,

I suspect you could use the current loop output trransistor as this 
inverter (see my previous message). And I suspect that if you buffered 
the RS232 output from you PC (using, say, a 1489), and fed that into the 
current loop input -- pin 12 of J7 -- and cut R1 :-) then that would work 
too. Or invert the output of the 1489 (say using a '04 -- or even another 
secion of the 1489 as a kludge :-)) and feed it into the SID pin of the 
8085 with the  link S25 totally removed. 

I would think that 110 bard would be trivial to do, and for the amount of 
data you need to transfer, not a real problem. The monitor is 2K, 110 
bard is 10 characters/second, so that's just oaver 200 seconds. Under 4 

> timed by busy loops in the firmware but that's not a problem either.
> There's a routine in the ROM called "co" which will bit-bang chars out
> the interface as well as one called "nmout" which outputs ascii-hex bytes=

Or you could run the tty monitor and use the 'D' command to dump the 
approrpatie area of memory (0 to 7FF I think). Capture the incoming data 
on the PC (I think any reasonable terminal progam can do that -- C-Kermit 
certainly can) and write a simple program to decode the hex dump back to 


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