diff --git a/src/mame/drivers/hp9k_3xx.cpp b/src/mame/drivers/hp9k_3xx.cpp
index a345873171f..4b16c8b390d 100644
--- a/src/mame/drivers/hp9k_3xx.cpp
+++ b/src/mame/drivers/hp9k_3xx.cpp
@@ -468,6 +468,7 @@ MACHINE_CONFIG_START(hp9k3xx_state::hp9k300)
 	MCFG_DEVICE_ADD(PTM6840_TAG, PTM6840, 250000) // from oscillator module next to the 6840
 	MCFG_PTM6840_EXTERNAL_CLOCKS(250000.0f, 0.0f, 250000.0f)
 	MCFG_PTM6840_O3_CB(WRITELINE(PTM6840_TAG, ptm6840_device , set_c2))
+	MCFG_PTM6840_IRQ_CB(INPUTLINE("maincpu", M68K_IRQ_6))
 
 	SPEAKER(config, "mono").front_center();
 	MCFG_DEVICE_ADD(SN76494_TAG, SN76494, SN76494_CLOCK)