Instruction Manual

ISO-16 User Manual

Relative Addressing vs. Absolute Addressing

The SeaIO API makes a distinction between “absolute” and “relative” addressing

modes. In absolute addressing mode, the Port argument to the API function acts as a

simple byte offset from the base I/O address of the device. For instance, Port #0

refers to the I/O address base + 0; Port #1 refers to the I/O address base + 1.

Relative addressing mode, on the other hand, refers to input and output ports in a

logical fashion. With a Port argument of 0 and an API function meant to output

data, the first (0

) output port on the device will be utilized. Likewise, with a Port

argument of 0 and an API function designed to input data, the first (0

) input port of

the device will be utilized.

In all addressing modes, port numbers are zero-indexed; that is, the first port is port

#0, the second port is #1, the third #2, and so on.

Direct Hardware Control

In systems where the users program has direct access to the hardware (DOS) the

table below gives the mapping and functions that the ISO-16 provide. The address of

each eight-bit port is calculated as shown in the table on the following page, the

cards base address plus an offset.

Reading the Inputs

The inputs are active Low. If no voltage is applied across one of the differential

inputs it returns a one on that bit. If an AC or DC voltage (of sufficient magnitude,

covered above) is applied it returns a zero on that bit.

Function Available Port Address Hex

R A Base + 0

R B Base + 1

R = Read

Address Mode D7 D6 D5 D4 D3 D2 D1 D0

Base+0

R/W PAD7 PAD6 PAD5 PAD4 PAD3 PAD2 PAD1 PAD0

Base+1

R/W PBD7 PBD6 PBD5 PBD4 PBD3 PBD2 PBD1 PBD0

Base+2