Optional ReadonlymaxCoils one FC01 transaction may return.
MODBUS_MAX_READ_COILS (2000)
Optional ReadonlymaxUnrequested coils the planner may read to join two spans into one.
CAUTION: A gap can cover an address the device does not implement. Such a
span fails with ILLEGAL_DATA_ADDRESS and takes every address in it down.
Options for planCoilReads.