modbus exception 2 for LJTick-DAC at startup

[kmpeters]

The LJTick-DAC register is write-only, so using a non-zero value for the pollMsec argument to drvModbusAsynConfigure results in the modbus exception 2:

# 2 additional analog outputs, setup to access LJTick-DAC occupying FIO0/FIO1
drvModbusAsynConfigure("LJT7_1_AO_2", "LJT7_1", 0, 16, 30000, 2, 8, 1, "LJT7_module")
2022/04/05 09:00:54.445 drvModbusAsyn::doModbusIO port LJT7_1_AO_2 Modbus exception=2

Changing pollMsec to zero eliminates the error.