Black Box IC171C - Manual (Page 27)

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RS422/485 SERIAL INTERFACE PLUS
The RS422/485 Serial Interface Plus is setup to use the AT interrupts, but
there’s no interrupt response.
How to identify the problem: Verify that the interrupt jumpers are correctly
set. Verify that the card is inserted into a 16-bit or AT slot.
Solution: Use the program SLT (found on your utility diskette) to verify that
the interrupts are set correctly. (Note that SLT requires that the transmit and
receive signals on the connector be “looped back” or connected together.
This can be accomplished by setting SW2 4 and 5 ON.)
Reason: AT IRQs are not found on the 8-bit slots only on 16-bit slots. The
higher level interrupts can never be used in a PC/XT compatible system or if
the board is installed in an 8-bit slot.