On Tue, 11 Jun 2019 10:33:58 -0700 (PDT),
firstname.lastname@example.org declaimed the following:
A must go to B and B must go to A between the two devices.
The correct way to hook up RS485 devices is to daisy-chain all the A's
together and all the B's. I believe that the Comms cape is simply
This may derive from a reversal in the schematic or something.
OR... from https://en.wikipedia.org/wiki/RS-485
The truth tables of most popular devices, starting with the SN75176, show
the output signals inverted. This has caused much confusion since, with
most device manufacturers following the SN75176's lead.
These names are all in use on various equipment, but the actual standard
released by EIA only uses the names A and B. The B line is positive
(compared to A) when data is 1. However, due to the ambiguous standard
(which refers to the A line as "non-inverting"), there is much confusion
about which is which. The RS-485 signaling specification, confusingly, says
that signal A is the non-inverting pin and signal B is the inverting
pin. This is in accordance with the A/B naming used, incorrectly, by
most differential transceiver manufacturers, including, among others:
Texas Instruments, as seen in their application handbook on EIA-422/485
communications (A=non-inverting, B=inverting)
Intersil, as seen in their data sheet for the ISL4489 transceiver
Maxim, as seen in their data sheet for the MAX483 transceiver
Linear Technology, as seen in their datasheet for the LTC2850, LTC2851,
Analog Devices, as seen in their datasheet for the ADM3483, ADM3485,
ADM3488, ADM3490, ADM3491
FTDI, as seen in their datasheet for the USB-RS485-WE-1800-BT
These manufacturers are all incorrect (but consistent), and their practice
is in widespread use, so care must be taken when using A/B naming.