don't want to fry another processor board

[Dan]

I'm working on a sync serial interface between 2 processors using 2 data and
1 clock. I had to replace 1 board because I was sending data while it was
off. By sourcing or sinking one of the IO pins, I fried the board. I'm
trying to think of a good way to avoid this. What I'm thinking of is using a
octal (or quad) latch which buffers the IO and is controlled by the 'master'
processor via the OC or enable pin. As long as the output of the latch is
hi-Z then I shouldn't have a problem. Right?

dansteely2001 at yahoo

 

[martin griffith]

On Wed, 12 Jan 2005 11:19:29 GMT, in sci.electronics.design "Dan"
<danNOSPAMsteely2001@yahoo.com> wrote:

I'm working on a sync serial interface between 2 processors using 2 data and
1 clock. I had to replace 1 board because I was sending data while it was
off. By sourcing or sinking one of the IO pins, I fried the board. I'm
trying to think of a good way to avoid this. What I'm thinking of is using a
octal (or quad) latch which buffers the IO and is controlled by the 'master'
processor via the OC or enable pin. As long as the output of the latch is
hi-Z then I shouldn't have a problem. Right?

dansteely2001 at yahoo

Just put 220R or so resistors in series with the port pins, that will

limit the current, and you can then scope either end to see if there
are any conflicts


martin

Serious error.
All shortcuts have disappeared.
Screen. Mind. Both are blank.

 

[CBarn24050]

Subject: don't want to fry another processor board
From: "Dan" danNOSPAMsteely2001@yahoo.com
Date: 12/01/2005 11:19 GMT Standard Time
Message-id: <5J7Fd.191694$8G4.168682@tornado.tampabay.rr.com

I'm working on a sync serial interface between 2 processors using 2 data and
1 clock. I had to replace 1 board because I was sending data while it was
off. By sourcing or sinking one of the IO pins, I fried the board. I'm
trying to think of a good way to avoid this. What I'm thinking of is using a
octal (or quad) latch which buffers the IO and is controlled by the 'master'
processor via the OC or enable pin. As long as the output of the latch is
hi-Z then I shouldn't have a problem. Right?

Alternatively you could just use a couple of diodes and a resistor.

 

[mc]

I'm not sure what caused the failure, but I'd suggest putting 1k resistors
between them instead of direct connections. CMOS data goes through a 1k
resistor just fine, and there's a lot less of risk of damage from accidental
connections or static.

"Dan" <danNOSPAMsteely2001@yahoo.com> wrote in message
news:5J7Fd.191694$8G4.168682@tornado.tampabay.rr.com...

I'm working on a sync serial interface between 2 processors using 2 data
and
1 clock. I had to replace 1 board because I was sending data while it was
off. By sourcing or sinking one of the IO pins, I fried the board. I'm
trying to think of a good way to avoid this. What I'm thinking of is using
a
octal (or quad) latch which buffers the IO and is controlled by the
'master'
processor via the OC or enable pin. As long as the output of the latch is
hi-Z then I shouldn't have a problem. Right?

dansteely2001 at yahoo

 

[Tam/WB2TT]

"Dan" <danNOSPAMsteely2001@yahoo.com> wrote in message
news:5J7Fd.191694$8G4.168682@tornado.tampabay.rr.com...

I'm working on a sync serial interface between 2 processors using 2 data
and
1 clock. I had to replace 1 board because I was sending data while it was
off. By sourcing or sinking one of the IO pins, I fried the board. I'm
trying to think of a good way to avoid this. What I'm thinking of is using
a
octal (or quad) latch which buffers the IO and is controlled by the
'master'
processor via the OC or enable pin. As long as the output of the latch is
hi-Z then I shouldn't have a problem. Right?

dansteely2001 at yahoo


Why not use a real RS232 transceiver. Maxim makes at least 1 with 3

drivers/3 receivers in one pack - single supply with built in charge pump. I
think these are available in SM only. Motorola made an (MC145407?) that had
several drivers/receivers in one 20 pin DIP package, 5V only, but I don't
see it at the ON web site. I don't know what bit rate you are running, but
these devices are good up to about 250 Kbs.

Tam

 

[Tim Shoppa]

As long as the output of the latch is
hi-Z then I shouldn't have a problem. Right?

If you pick an arbitrary tri-state latch, you might end up with one
that is high-Z when there is no Vcc. Or you might end up with one that
shorts to ground. With most CMOS parts, any voltage you put on a
powered-down chip finds its way to Vcc and attempts to power the whole
circuit. It's a handy trick for when you need it, but it's probably
what's doing you in right now.

If you use a part specifically designed to be high-impedance when
powered down then it's not a big deal... but I don't know of any
octal/quad tri-state latches that fall in this category. Usually these
are things like RS-232 or RS-423 transceivers (which are not all that
high impedance, but are usually better than a few K ohms.)

Tim.

 

[Dan]

Why not use a real RS232 transceiver. Maxim makes at least 1 with 3
drivers/3 receivers in one pack - single supply with built in charge pump.
I
think these are available in SM only. Motorola made an (MC145407?) that
had
several drivers/receivers in one 20 pin DIP package, 5V only, but I don't
see it at the ON web site. I don't know what bit rate you are running, but
these devices are good up to about 250 Kbs.

Tam

That makes a lot of sense. I've used these for async serial but there's no
reason they won't work with sync serial.

 

[Tam/WB2TT]

"Dan" <danNOSPAMsteely2001@yahoo.com> wrote in message
news:NKbGd.237132$6w6.89833@tornado.tampabay.rr.com...

Why not use a real RS232 transceiver. Maxim makes at least 1 with 3
drivers/3 receivers in one pack - single supply with built in charge
pump.
I
think these are available in SM only. Motorola made an (MC145407?) that
had
several drivers/receivers in one 20 pin DIP package, 5V only, but I
don't
see it at the ON web site. I don't know what bit rate you are running,
but
these devices are good up to about 250 Kbs.

Tam

That makes a lot of sense. I've used these for async serial but there's no
reason they won't work with sync serial.


I can't remember if it is part of the RS232 spec, but the original EIA

specification actually had pins assigned to the clock leads. If you go back
to before UARTs were invented in the 1970s, all modems that I had seen that
ran faster than 300 Baud were synchronous. Just to show how things have
changed, we once had a Western Electric 103A 300 Baud modem. It was the size
of a large DVD player, with about 10 plug in boards.

Tam