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Using inverter to drive ATN for RS-232

 
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spamiam



Joined: 13 Nov 2005
Posts: 666
Posted: 02 August 2010, 23:35 PM    Post subject: Using inverter to drive ATN for RS-232 Reply with quote
I have an M328p and I am working on external support circuits. I looked at the recommended circuits, and I see that it is possible to drive the RXD and TXD lines with inverters. But the ATN line always needs a discrete transistor. Since I would have a few spare inverters available if I use the inverter method to interface the RXD and TXD signals, I would love to be able to use an inverter for the ATN signal.

Since there is no recommended inverter-based circuit for the ATN signal, I presume it is just not possible. But, might there be an inverter circuit for the ATN too? The ATN circuits looks rather similar to the transistor-based RXD circuit, so I thought it might be feasible....

-Tony
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dkinzer
Site Admin


Joined: 03 Sep 2005
Posts: 2499
Location: Portland, OR
Posted: 03 August 2010, 0:55 AM    Post subject: Re: Using inverter to drive ATN for RS-232 Reply with quote
spamiam wrote:
But, might there be an inverter circuit for the ATN too?
Sure, you can use a simple inverter instead of the transistor. The advantage of the transistor circuit is that you can feed alternate reset signals to the reset pin since the pin isn't being driven high.

If you need the ability to have other reset sources, you can still use an inverter by adding a diode to isolate the inverter's high driver from the reset signal. For example, see the attached image.

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spamiam



Joined: 13 Nov 2005
Posts: 666
Posted: 03 August 2010, 2:19 AM    Post subject: Re: Using inverter to drive ATN for RS-232 Reply with quote
dkinzer wrote:
you can still use an inverter by adding a diode to isolate the inverter's high driver from the reset signal. For example, see the attached image.


I figured that I would need a diode in series, but I was not sure that the transient responses would be a sufficient match for the transistor responses.

I just looked thru my box-o-parts and I don't see my 74 series parts. I do see my CD4000 series parts. Those things are pretty slow. Do you think that these will be fast enough for this application? I suspect the slew rate is not sufficient for 115200 baud.

-Tony
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dkinzer
Site Admin


Joined: 03 Sep 2005
Posts: 2499
Location: Portland, OR
Posted: 03 August 2010, 2:22 AM    Post subject: Re: Using inverter to drive ATN for RS-232 Reply with quote
spamiam wrote:
Do you think that [CD4000 parts] will be fast enough for this application?
I suspect that they might be too slow for 115K baud but you can hook one up and look at the output on the scope while transmitting U continuously. At N81, this produces a square wave.
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spamiam



Joined: 13 Nov 2005
Posts: 666
Posted: 03 August 2010, 11:55 AM    Post subject: Re: Using inverter to drive ATN for RS-232 Reply with quote
dkinzer wrote:
spamiam wrote:
Do you think that [CD4000 parts] will be fast enough for this application?
I suspect that they might be too slow for 115K baud but you can hook one up and look at the output on the scope while transmitting U continuously. At N81, this produces a square wave.


I just had another thought. Maybe I can use the second RX channel on the MAX232 that I am already using for the RXD and TXD channels, and similar to the inverter circuit, place a diode in series to the /reset pin? Do you think that this would work?

If this would work, then I have all the hardware I need already present on my current project! It is certainly easy to test because I only need to add a diode and a jumper to the /reset pin. Everything else is there.

If this would work, maybe this alternative could be added to the suggested circuits on the diagram showing the MAX232.

-Tony
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dkinzer
Site Admin


Joined: 03 Sep 2005
Posts: 2499
Location: Portland, OR
Posted: 03 August 2010, 15:13 PM    Post subject: Re: Using inverter to drive ATN for RS-232 Reply with quote
spamiam wrote:
Maybe I can use the second RX channel on the MAX232 that I am already using for the RXD and TXD channels, and similar to the inverter circuit, place a diode in series to the /reset pin? Do you think that this would work?
I see no reason that it wouldn't work.
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spamiam



Joined: 13 Nov 2005
Posts: 666
Posted: 09 August 2010, 3:14 AM    Post subject: Re: Using inverter to drive ATN for RS-232 Reply with quote
dkinzer wrote:
spamiam wrote:
Maybe I can use the second RX channel on the MAX232 that I am already using for the RXD and TXD channels, and similar to the inverter circuit, place a diode in series to the /reset pin? Do you think that this would work?
I see no reason that it wouldn't work.


I can confirm that it works fine. It requires just the diode as you indicated. I did not use a capacitor for AC coupling.

I think that when using something like a MAX232 which has 2 RX channels, then it makes good sense to use the second RX channel for the ATN signal. It definitely reduces the parts count compared to the suggested circuit incorporating the MAX232.

-Tony
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