Motorola and V.32bis - Manual (Page 225)

Country-Specific Information C-45
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Information
Refer also to Figure C-10, which shows pin locations on Modulus 21 backplanes.
Modulus 9s have a similar layout.
Figure C-10. Modulus Backplane Pin Locations (Modulus 21 Shown)
In Hong Kong and the United Kingdom, Busy Out backplanes are hard-wired for
Busy Out operation. These backplanes do not use pin jumpers.
Jumpers on
Left Pin Pairs
Pins
Pins
Modem A
Modem B
Modulus 21 Enclosure,
Rear View