Whenever an IOCtl command calls for a NULL pointer, it is the responsibility of the application to set one up for the appropriate Parameter or Data Packet pointer before calling the physical device driver. IOCtls can be interpreted differently by future releases if the pointer is not a NULL pointer. If a NULL pointer is called for and it is not received by the device driver, it is considered an invalid Parameter or Data Packet value.
The physical device driver services each communications port (COM1, COM2, and so forth) independently. IOCtls issued to the physical device driver for a given port have no effect on any other communications ports that the physical device driver is servicing. The application cannot assume a given timing relationship between when the IOCtls are executed and when data is received or transmitted by the ASYNC hardware. Data Carrier Detect (DCD) is the same signal as Receiver Line Signal Detect (RLSD).
The following is a summary of the Category 01h IOCtl Commands:
┌──────────┬──────────────────────────────────────────────────┐ │Function │Description │ ├──────────┼──────────────────────────────────────────────────┤ │ 14h │Reserved │ ├──────────┼──────────────────────────────────────────────────┤ │ 34h │Reserved │ ├──────────┼──────────────────────────────────────────────────┤ │ 41h │Set Bit Rate │ ├──────────┼──────────────────────────────────────────────────┤ │ 42h │Set Line Characteristics (stop, parity, data bits)│ ├──────────┼──────────────────────────────────────────────────┤ │ 43h │Extended Set Bit Rate │ ├──────────┼──────────────────────────────────────────────────┤ │ 44h │Transmit Byte Immediate │ ├──────────┼──────────────────────────────────────────────────┤ │ 45h │Set Break OFF │ ├──────────┼──────────────────────────────────────────────────┤ │ 46h │Set Modem Control Signals │ ├──────────┼──────────────────────────────────────────────────┤ │ 47h │Behave as if XOFF Received (stop transmit) │ ├──────────┼──────────────────────────────────────────────────┤ │ 48h │Behave as if XON Received (start transmit) │ ├──────────┼──────────────────────────────────────────────────┤ │ 49h │Reserved │ ├──────────┼──────────────────────────────────────────────────┤ │ 4Bh │Set Break ON │ ├──────────┼──────────────────────────────────────────────────┤ │ 53h │Set Device Control Block (DCB) Parameters │ ├──────────┼──────────────────────────────────────────────────┤ │ 54h │Set Enhanced Mode Parameters │ ├──────────┼──────────────────────────────────────────────────┤ │ 61h │Query Current Bit Rate │ ├──────────┼──────────────────────────────────────────────────┤ │ 62h │Query Line Characteristics │ ├──────────┼──────────────────────────────────────────────────┤ │ 63h │Extended Query Bit Rate │ ├──────────┼──────────────────────────────────────────────────┤ │ 64h │Query COM Status │ ├──────────┼──────────────────────────────────────────────────┤ │ 65h │Query Transmit Data Status │ ├──────────┼──────────────────────────────────────────────────┤ │ 66h │Query Modem Control Output Signals │ ├──────────┼──────────────────────────────────────────────────┤ │ 67h │Query Current Modem Input Signals │ ├──────────┼──────────────────────────────────────────────────┤ │ 68h │Query Number of Characters in Receive Queue │ ├──────────┼──────────────────────────────────────────────────┤ │ 69h │Query Number of Characters in Transmit Queue │ ├──────────┼──────────────────────────────────────────────────┤ │ 6Dh │Query COM Error │ ├──────────┼──────────────────────────────────────────────────┤ │ 72h │Query COM Event Information │ ├──────────┼──────────────────────────────────────────────────┤ │ 73h │Query Device Control Block (DCB) Parameters │ ├──────────┼──────────────────────────────────────────────────┤ │ 74h │Query Enhanced Mode Parameters │ └──────────┴──────────────────────────────────────────────────┘