3.4 The channel registers

Each of the four channels occupies 4 addresses.

Channel Registers
channel offset	Read	Write
1	BIM Control	BIM Control
9	BIM Vector	BIM Vector
11	Current State	Compare State
19	Change State	-

The first address is the control register supplied by the BIM. It is used to set the interrupt level and the interrupt enable bits of a channel.

The second address is the vector register supplied by the BIM. The Current State Register supplies the current state of the input signals. An input voltage lower than 4 Vdc on the input port is read as a '0'. An input voltage higher than 17 Vdc on the input port is read as a '1'.

The Current State Register is a read only register. The following table shows the Current State Register bit assignment for the four channels.

Current State Register Bit
Channel	Bit	Input
0	0 .. 7	0 .. 7
1	0 .. 7	8 .. 15
2	0 .. 7	16 .. 23
3	0 .. 7	24 .. 31

The Compare State Register is used by the Change of State Circuit. The contents of this register is continuously compared with the state of the inputs. When a mismatch is detected an interrupt request is generated to the BIM. The Compare State Register is a write only register.

The contents of the Change State Register is set by the Change of State Circuit. It holds the 8-bit image of the state of the inputs which caused the Change of State interrupt. The Change State Register is a read only register.

CC92 - Isolated Digital Input Board - 3 OCT 1996

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