RS485 Interface board for Propeller - Updated to REV-P

DavidM

Hi,

I have updated my PCB design ( and schematic) again , the Diode D2 was in the wrong place, And I have moved the transformer closer to the IC.

Any comments on my design?

Q1) If I were to connect 3 of these RS485 boards ( using the prop) with only say about 2 meters of cable between nodes 1 & 2 & 3, according to my design both end nodes ( 1 & 3) will be terminated, but I have read that short lines do not need termination, so does this mean it won't work?

Q2) Do I need the resistor R17 to be placed as i have done, as part of the DIODE regulator circuit? is it the correct value/wattage?

Q3) I have chosen a different OPTO COUPLER since the last design, will this work?

Q4) Is 1 watt for the resistors R2, R4 & R5 to large? or will 1/2 watt work just fine?




Thanks

Dave M

Post Edited (DavidM) : 4/20/2009 11:04:43 AM GMT

dMajo

Hi DavidM,

· I am still busy (sorry, the revamping is taking more time that I expected) so I have given just a quick look. I assume that the schematic was not changed and that the pcb correspond to it (I haven't looked for routing errors)

A1) "it doesn't need" it doesn't mean "it doesn't owe": it is always good to terminate properly the lines. If the lines are short, you have less reflections/distorsions and it can work even without termination (for low speeds)
A2) No, you not need and you not owe: D2 is just for precaution (to eventually protect your AHC logic), usually it will never act (anyway it can handle the current from the transformer also without R17). With R17=10R and let's say I(Vcc2)=200mA you will loose 2V on this resistor and this is not good
A3) PS2581AL2 is OK (R11=6K8 to 8K1; R10=560), PS2597AL2 not able to find datasheet right now
A4) 1/2W can do the job if everything OK but for safety reasons (product life time) I suggest you 1W

The PCB at first look seems OK. Not so easy to hand-solder because of the reduced space between some components. Take care at mounting screws: you do not want to make a short between the two grounds (at least one ground plane maybe too close to screws holes)

regards

DavidM

Thanks dMajo,

I have updated the schematic & the layout.

I removed the resistor for the diode.
I consolidated some capacitors ( the 10UF ones)
I have updated the BOM which is now attached along with the layout & schematic.

I have to work out how to make more space around the HOLES

thanks

Dave M

DavidM

HI,

I have made some changes to the PCB Layout.

1) I removed the bottom 2 mounting holes , these are not needed as the connectors will hold the board in place ( these connector are insulated).

2) I moved C6 as downwards, and moved the TRANSFORMER a Little closer to the IC.

3) The tracks for the R4/R5 have been increased.

4) If only the EXTERNAL LED's are used ( via the 4 pin header) then , no need to mount the internal ones. So only one set of resistors is needed. The internal leds are for testing, the external ones are for mounting on a panel.

The revision is now REV-P

regards

Dave M