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Microchip Davidtait Topiclpt

                LPT Port Assumptions for TOPIC

                          David Tait


This short file describes the assumptions made about the printer port
(LPT) hardware used to control the TOPIC programmer.  It seems that
these assumptions may not be correct for some PCs.  In particular new
PCs equipped with enhanced ports as defined in the recent IEEE 1284
standard.  These ports should be capable of working in standard
printer port mode - "compatibility mode" - but it appears some are
more compatible than others.  Unfortunately, where I assume a
particular printer line is controlled by an open-collector (O/C)
driver it seems some ports use "totem-pole" drivers instead (the
former can only sink current but the latter can both sink and source
current).  The differences mean that the TOPIC hardware may not work
on these systems without modification.


It is assumed that the INIT line is controlled by an open-
collector driver.  This is not too critical but if INIT is
actually controlled by a totem-pole output it may be necessary 
to reduce R4 from 1k to 470 ohms or even lower.  Just check that
the output of the LM317L reaches +13.5V when INIT is low.

It is assumed that the STROBE and AUTOF lines are controlled by 
open-collector drivers.  This is handy because they drive RB6 and
RB7 which are Schmitt trigger inputs in programming mode; the
O/C drivers are better able to reach the input high threshold of 
nominally 4V.  If the O/C assumption is wrong here it's unlikely
to be a problem as new ports will probably have an adequate high 
output voltage (that's because they are usually implemented using 
CMOS VLSI).  I don't use the LPT data lines (totem-pole outputs) to 
drive RB6 and RB7 as doing so might mean TOPIC will not work with
an old port based on traditional TTL.

It is assumed that SEL (actually SELECTIN) is controlled by an 
open-collector driver.  This line has two functions.  It is used 
to check whether the programmer is plugged into the PC: As SEL is 
connected to the BUSY input on the TOPIC board the line can be
toggled and the effect on BUSY monitored and thus used to verify 
the programmer is attached.  The second function is more subtle
and is explained along with the description of the next assumption.
I'll leave a discussion of the consequences of SEL really being a 
totem-pole output until later.

It is assumed that the BUSY input has an internal pull-up resistor.
This assumption is necessary because BUSY is connected to RB7 via
a diode and RB7 can only pull BUSY low.  You might ask why the diode 
is required - if it wasn't fitted there would be no need to assume
BUSY has a pull-up.  OK, let's assume there is no diode between
BUSY and RB7.  When reading from the PIC everything is fine but
consider what happens when writing to the PIC.  I have already
said that I assume AUTOF has an O/C driver and, to prevent damage
when the PIC is running with RB7 enabled as an output, it is
connected to RB7 via a 1k resistor.  When AUTOF is high RB7 is 
connected to +5V via the combination of the 1k resistor and the 
AUTOF pull-up and it is also connected to +5V via the BUSY pull-up;
this pretty well guarantees RB7 is high.  Now, when AUTOF is low
RB7 is grounded via the 1k resistor but it is still connected to
+5V via the BUSY pull-up.  This means the voltage at RB7 depends
on the value of the BUSY pull-up - if the pull-up is 4.7k the
voltage will be around 0.9V and will be even greater if the
pull-up has a lower resistance.  This value is too close to the low
threshold for comfort.  By adding the diode the voltage drops
slightly but could still be too high.  However, remember SEL is also
connected to BUSY; by programming SEL low the influence of the
BUSY pull-up is removed (this is the real reason for using the diode:
without it programming SEL low would simply short RB7 to ground).

When reading from the PIC the SEL line is pulled high.  But consider 
what happens if SEL really has a totem-pole output.  In this case 
SEL will "fight" with RB7 when trying to read from the PIC.  It is 
likely that the PIC will win (it has a fairly beefy output driver) 
but this is not a good situation.  To get around this SEL can be made 
into a pseudo O/C driver by adding another diode; the diode replaces
the SEL/BUSY short with the cathode connected to SEL and the anode 
to BUSY: 

        SEL  ------------+          +------------ RB7
                         |          |
                         |          |
                        ---        ---
              new diode / \        / \ existing diode
                        ---        ---
                         |          |
                         |          |
        BUSY ------------+ ---------+

Although I assume BUSY has a pull-up it doesn't really matter if 
it doesn't because, provided it is O/C, SEL will.  However if the 
diode is added as described above then BUSY _must_ have a pull-up. If 
it doesn't, a resistor (say 4.7k) from BUSY to +5V should be fitted to 
the TOPIC board.


If the TOPIC board doesn't work with your LPT port try: 1) changing R4 
from 1k to 470 ohms; 2) adding a diode between SEL and BUSY.  If it 
still doesn't work add a 4.7k resistor from BUSY to +5V.  If it still
doesn't work let me know (tell me you've read this document).


Thanks to Kalle Reunanen for his comments about the value of R4.  Thanks
to Bob Parker for telling me about the "extra diode" fix.


file: /Techref/microchip/davidtait/topiclpt.txt, 5KB, , updated: 2001/4/17 11:15, local time: 2018/1/23 08:18,

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