RS485 Adapter raspberry pi non isolated

This is a basic RS485 interface board for the raspberry pi model A, it uses the 26 way connector, it could also be used on later versions by only fitting a header to the top 10 pins to include the Uart and power connections

Non isolated RS485 board for the raspberry pi
Non isolated RS485 board for the raspberry pi Click to enlarge

 

 

The 555 timer U2 (Make sure to use a CMOS one that works to <3.3V) works as a monostable set for around 40 us(Via R3 & C3), every time the TXD pin goes low it resets the timer via D1 and when it goes high the monostable triggers, after the monostable period it disables the transmitter but the lines A&B are still pulled apart (Representing a 1) by R2 & R4. In this way the transmitter is automatically disabled within 40us of the last low to high transition on the TXD pin. The operation can be seen in the scope plot below the blue plot is the DE input and the yellow trace is the TX input, shortly after the last low to high transition the DE input disables the transmitter.

IMG_0224

 If automatic enable isn’t needed(RS485 DE) then Jumper J7 can be removed and J9 inserted then GPIO 17 can be used to drvie the DE/RE pin To select thdifferent modes of operation below is a table showing which jumpers should be fitted, Iso Table

The board is a 2 layer board the diagram below shows the placement of the jumper connections

RS485 Adapter Layout
RS485 Adapter Layout

This board is compatible with the original Model A & B Raspberry Pi’s with the 26 way connector

Compatibility with Raspberry Pi Models

The bill of materials for the project is

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                             SP3483E
1                    U2                             555(Make sure it works at 3.3V)
1                    U3                             ADP122

Diodes
——
1                    D1                             1N4148

Miscellaneous
————-
1                   CN1                            CONN-DIL26
1                   J1                                TBLOCK-I3
6                   J2-J4,J7-J9                  CONN-H2