Tiva TM4C123 not receiving CAN message

Hi I'm trying to test CAN functionality by having two Tivas communicate. One is flashed with simple_rx, and one with simple_tx from the tivaware examples.  Each is connected to an ISO1050 transceiver. The only change I've made to the code is set the clock to 80 MHz. The tx Tiva repeatedly prints "error - cable connected?" and the message received flag on the rx never gets set. Not sure where to go next in debugging, any help would be appreciated.