I have been digging into this a bit, and what I am discovering is that there are some cables that are not RS-232 compliant. What this means is that the voltage swing is not +/-10V but much, much smaller. I know that the MAX3221 device needs to operate within a +/- 2.5V swing range - it needs to see this level on the receive side (i.e. from PC to uS), otherwise it will go into auto-shutdown mode on the transmit side (from uS to PC).nemiro wrote:It could be, but also check which USB-> serial cable you are using. I have found on another uS that it was VERY picky about what cable I needed to use. I ended up having to spring for the high dollar cable, and everything worked fine.
From the Maxim datasheet:
Autoshutdown works by monitoring the RS-232 side of the receivers. When connected to another RS-232 device, the receivers will see valid RS-232 signals that are either below -3V or above +3V. On the other hand, if nothing is connected, the receivers will typically be at ground. If the Autoshutdown feature detects that all the receivers are between -.3V and +.3V for greater than 30µS, it assumes that a valid transmitter is not connected and will automatically put itself into a low-power mode. The part will automatically take itself out of the low-power mode if any one of the receivers' inputs exceeds +2.7V or goes below -2.7V.
So - your computer needs to be supplying a valid RS-232 level. I know that the USB-to-Serial cable I use has a "Prolific" chipset and it provides RS-232 signalling (I scoped this myself). The one I have is the brand name of Sewell and can be purchased for $12.00 online from several sources.
- Bruce
