IBM RT Series Hardware Reference Manual page 62

I/O Decode (In Hex)
porte
Register Selected DLAB State
n240 TX Buffer DLAB=O (Write)
n240 RXBuffer DLAB=O (Read)
n240 Divisor Latch LSB
DLAB=l
n241
Divisor Latch MSB DLAB=l
n241 Interrupt Enable Register DLAB=O
n242 Interrupt Identification
Register
n243 Line Control Register
n244 Modem Control Register
n245 Line Status Register
n246 Modem Status Register
Figure 6. I/O Decodes, Port C
Notes:
1. n is equal to the first digit of the adapter address range
2. DLAB means Divisor Latch Access Bit.
6 5080 Peripheral Adapter