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Combied RX/TX Line

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rezer

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I'm thinking you could try this chip. It's a quad buffer chip and you only need one, but it should work.
http://www.electro-tech-online.com/custompdfs/2008/09/74AHC_AHCT126_3.pdf

If you tie your TX line to the active high enable replace the diode with the buffer, then that should block your TXD from being echoed to the RX line. My thinking is, the TXD line, while transmitting, when it's at the mark state, will enable the buffer, but since the RX line is at the mark state, it won't have any effect. When the TX line drives low (space), you see that the buffer will be disabled, blocking the TX signal and keeping the RX line at the mark state. While receiving data, since it's half duplex, the TX line being at the mark state will keep the buffer enabled and ready to receive data onto RX. The TX line should force enable/disable just prior (ns) to the TX data reaching the output. This should prevent any spurious signals from getting through to the RX line. You could use an RC circuit of 470Ω/1000pF in line with the input buffer to provide a short delay for the RX data. It should't have any effect as long as your baud rate is reasonable.
 

rezer

New Member

rezer

New Member
Usually they have a programmable loop back mode for testing. Is that turned off?
Are you talking about the buffer IC? If so, there isn't any loop-back mode on this device. Please explain.
 

Space Varmint

New Member
Are you talking about the buffer IC? If so, there isn't any loop-back mode on this device. Please explain.
Well maybe I'm not sure what you are talking about but the NS16450 and it's predecessors have a programmable loop back feature which will send back the data it receives. They are a full duplex chip and you have to turn that feature off.
 

rezer

New Member
Well maybe I'm not sure what you are talking about but the NS16450 and it's predecessors have a programmable loop back feature which will send back the data it receives. They are a full duplex chip and you have to turn that feature off.
When was the NS16450 UART mentioned? I looked through the posts, but saw nothing. Maybe this is what he is using to serialize his data, I don't know. Where did you see it mentioned?

Anyway, were were talking about using a buffer with active low enable in place of his diode. The issue was, because the TX and RX lines share the same output, the TX signal was being passed, or "echoed" back to the receive input. I don't think loopback is his issue (Well, technically it is;)).
 

Scarr

Member
No usart involved

Hi Rezer / space varmit,

Rezer, I'm on with board and ordered the MC74VHC1GT126 and will post when results are in.

Thx
 
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