Panasonic JB-3300 Technical Manual page 220

(3) Current loop circuit
XCDT
Ri
49.90
9
HO) XCDR
Figure 5-71 Transmit Circuit
+5V
Figure 5-72 Receive Circuit
(4) Interrupt circuit
Set Bit 3 of the MODEM control register to "1" to enable an
interrupt request from 8250. Interrupt requests are produced to
the interrupt controller by IRQ4.
NOTE: Either of the interrupt level IRQ3 or IRQ4 may be set by
means of a short-circuit plug (J6). When the equipment
is shipped, the interrupt level is set to IRQ4.
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