ETHERNET TO RS-485

The Coyote RS-485 interface is a two-wire, half-duplex configuration. My RS-485 card in the RS-485 PC is a four-wire, full-duplex configuration. So, I had to do some more cabling.

After checking the pinout diagram, I saw that the labels for the RS-485 port (J9) matched the pin designations. To turn the PC’s four-wire interface into a two-wire interface, I tied the RS-485 “A,” or negative, transmit and receive pins together, and then did the same for the “B,” or positive, transmit and receive pins.

To keep the wiring tangle factor low, I used a standard eight-pin RJ-45 female connector, Cat 5 cabling, and one of the Molex female headers and pins that comes with the Coyote. I punched down a short between the female RJ-45 connector’s pin 1 (TDB+) and 3 (RDB+), and 2 (TDA–) and 4 (RDA–).

Because the PC’s RS-485 interface is an RJ-45 female, I opted for a standard 568B-wired Cat 5 patch cable to connect the Coyote’s RS-485 interface to the RS-485 card. This placed my PC’s RS-485+ on pins 1 and 3, and the RS-485– on pins 2 and 4. I used pins 1 and 2 of the Cat 5 patch cable to carry the RS-485 signals and wired the corresponding pins into the Coyote’s J9 Molex connector.

I also added a connection on the J9 Molex connector ground pin to pin 8 of the Cat 5 patch cable, which is ground on the PC’s RS-485 card. Technically, I could run the RS-485 connection without a ground, but it’s always a good idea to include the third wire.

I used the same example program, serialexa.c, to build the Ethernet-to-RS-485 converter firmware. All I had to do was add a define block for the RS-485 port (serial port D) and insert the ser485Rx() and ser485Tx() functions ahead of every serial read (receive) and write (transmit) call in the serialexa.c code.

The ser485Rx() and ser485Tx() functions enable the functionality of the RS-485 transmit-enable signal that’s assigned to PA4 (i.e., I/O port A, pin 4). Because the Coyote is configured for Half Duplex mode, the ser485Xx() functions provide the code to enable and disable the RS-485 transmitter on the Coyote’s Sipex SP483 RS-485 driver IC via PA4. Otherwise, the data transfers resemble RS-232. A fragmented look at my modifications for RS-485 operation is shown in Listing 2. The RS-485 portion of the Ethernet-to-RS-485 converter session is shown in Photo 2. “Splash two.”