IBM RT User's Setup Manual page 173

6DMA channel 3 is only required when the PC Network Adapter is used with the Personal Computer AT Coprocessor Option.
*The AIX operating system restricts serial port A (2) to use of DMA channel 0; DMA channel 2 is used by the diskette adapter.
DMA channel 2 may be available when other operating systems are used.
8DMA channels 5 and 6 are not supported by IBM software.
AJZIF043
1-22 Options Installation