RS232 debug on JS16

[irob]

(Ugg, how embarrassing is this, right?)

So I'm getting the SCI port on my JS16 DEMO board off its feet.  This board has no RS232 transceiver on it, so I've got my own little add-on board with an ICL3232 and serial DB9 cable.  This cable has been proven to work with other UART projects.

I've got the SCI port enabled and transmitting, as I can verify with an o-scope on the transmit pin.  Nice strong +5V inactive, driven to ground when active.

When I plug in this transceiver, the transmit line gets loaded down and my JS16 can't drive all the way to ground (only about a volt).

Looking at just the transceiver, with its TTL TX and RX shorted together in loopback mode, I can transmit and see echo on a PC serial monitor.  So I know that the chip is alive and working independently of the JS16.

Why don't they work together?  I've checked pinouts and power and port pins twice over.

[bigmac · Accepted solution]

Hello irob,

The only thing that comes to mind is that you have the TXD output at the MCU connected to a receiver output at the ICL3232, giving an I/O conflict.

Note that it is likely that your device will require a DCE connection, since the PC provides a DTE connection, and I assume that you don't want to use a special null modem cable.  For DCE, the TXD line from the PC would need to connect, via a RS232 receiver, to the RXD pin of the MCU.  Similarly, the TXD pin of the MCU would connect to a RS232 driver, to provide the RXD line back to the PC.

Regards,

Mac

[irob]

bigmac, you were right.  I stared a little harder at my ICL chip datasheet and realized today that I had PC TXD line going to a Tout instead of a Rin.

Not sure what caused the cranial short circuit last week.  I don't know about you, but for me when I make a false assumption it can be very hard to force myself to take a fresh perspective and get out of the mental rut.

Thanks for the fresh perspective!