After the amount of interest seen in the isolated RS485 adapter and a few questions about the cost of the isolated transceiver, I decided to do a simplified non isolated layout that for application in non harsh environments should work fine. Please note though there is no isolation so any surges on the RS485 lines can go right through and damage the raspberry pi.

The Schematic:

Operation is very similar to the isolated converter

an 555 timer automatically disables the driver after

the end of transmission.

If J7 is fitted in this mode the pull up a pull down

resistors must be connected with J2 & J3.

The operation of the DE pin is shown in the scope

plot opposite  the light blue is DE and the yellow

the TX pin. when TXD = 0 the Driver is enabled.

At the end of transmission the driver is

automatically disabled after a short delay.

Alternatively the DE pin can be connected

to the GPIO pin 17 via fitting a jumper in J9.

The driver can be terminated by fitting J5.

The layout is a two layer PCB  as shown opposite

The bill of materials for this project is below:

QTY  PART-REFS           VALUE              

---  ---------           -----              

Resistors

---------

1    R1                  100                

2    R2,R4               560                

1    R3                  3.9K               

1    R5                  120                

2    R6,R7               10K                

 

Capacitors

----------

3    C1,C2,C7            10u                

2    C3,C6               10n                

1    C8                  100n               

 

Integrated Circuits

-------------------

1    U1                  ADM3483 (note schematic needs correcting)            

1    U2                  555 (ensure it works at 3.3V)                 

1    U3                  ADP122 (ensure 3.3V output version)            

 

Diodes

------

1    D1                  1N4148             

 

Miscellaneous

-------------

1    CN1                 CONN-DIL26         

1    J1                  TBLOCK-I3          

6    J2-J4,J7-J9         CONN-H2