View Full Version : Demo board schematic for prop plug has an error

08-31-2007, 05:35 AM
I used a USB to Serial converter to make the prop-plug shown in the demo board schematic but couldn't get it to actually identify a prop. I blamed by circuit and tried breadboarding another prop and experimenting. In the end it turned out to be the RX and TX lines being the reverse of what is needed.

The schematic shows TX on the USB chip going to P31 and the RX going to P30 while in reality those need to be reversed. Quite the relief for me really, since I went ahead and etched a little prop board without breadboarding first. Thought for a while there I had to make another!

Paul Baker
08-31-2007, 05:50 AM
It's a matter of reference, the TX and RX labeled on the USB chip are from the point of view of the USB chip, it sends data on the TX (connected to pin 31 of the Propeller) and it reads data from the RX (connected to pin 30 on the Propeller). Whereas the Propeller recieves data on pin 31 and sends data on pin 30. The top diagram of http://www.parallax.com/propeller/media.asp explicitly shows this.

Paul Baker (mailto:pbaker@parallax.com)
Propeller Applications Engineer
[/url][url=http://www.parallax.com] (http://www.parallax.com)
Parallax, Inc. (http://www.parallax.com)

Fred Hawkins
08-31-2007, 06:13 AM
I wish http://www.parallax.com/dl/docs/prod/prop/Propeller-ProtoBoard-v1.1.pdf was as clear about these details.

08-31-2007, 12:44 PM
As always, it depends from where you look http://forums.parallax.com/images/smilies/smile.gif The standards define there is a DTE ("Data Terminal Equipment") and a DCE ("Data Circuit terminating Equipment"): There terms "transmitting" and "receiving" have always to be used from the point of the DTE!

When you "transmit" something out of the PC it becomes somthing "received" at the USB-Plug/Prop Interface!


DTE ----- DCE ......... DCE ------ DTE


DTE ----- [ null modem = two headed DCE ] ------ DTE


Prop ----- USB-Plug ......... Cable-Plug ------ PC

Post Edited (deSilva) : 8/31/2007 5:53:12 AM GMT

08-31-2007, 10:32 PM
Yep, I'm aware that TX goes to RX and visa versa. What I meant was that when I hooked the wires up as shown it didn' work. I'm thinking now though, that the USB interface might be labelled opposite of what I expected with Its RX pin attached to the FT232's TX already.

09-01-2007, 02:04 AM
No Pat, it's not your fault! The USB plug is labelled wrong. It has no right to say it "receives" something from the Prop http://forums.parallax.com/images/smilies/smile.gif It works for the Prop as a DCE "transmitting" stuff. That at least would conform to the standards...