#05, M1S: RS485 Port
Function: RS485 port with full-duplex or half-duplex operation
Form factor: M1S
Category: Input/output module
Special needs: [SER]
Power requirements: 5V/15mA
Mates with: #19, #20, #21
See also: #01, #02
This port has only +/-RX and +/-TX signal pairs. If you are working in half-duplex mode, connect +RX to +TX and -RX to -TX. This will allow your system to communicate over a single twisted pair. Direction control is through the RTS line — the line must be LOW for data input and HIGH for output. Keep the RX and TX pairs separate for full-duplex mode and set the RTS line HIGH to keep the TX enabled at all times.
For "normal" RS485 applications, this Tibbit has to be connected to the TX and RX lines of the CPU's UART (see [SER]).
Combine this Tibbit with terminal block devices — #20 (nine terminal blocks) or #21 (four terminal blocks).
The original version of this Tibbit made no use of IO3 and IO4.
Revision B provides 5V power on IO3 and GND on IO4. There is also an onboard 120Ω termination resistor (0402 package) between the +/-RS485 lines. This resistor was added for the convenience of users.
There are three LEDs: two red and one green. The red LEDs are connected to the TX and OUT/-IN lines. The green LED is for the RX input. All LEDs are buffered (with logic gates) and light up for the LOW state of control lines. Revision B devices are marked by a small round sticker with the letter "B" on the side of the Tibbit.