IBM RT PC, Options Installation

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Section 6. Communication and Network Adapter Options 6-33
With the IBM RT PC 4-Port Asynchronous RS422A Adapter and the
correct cables, you can connect a system unit to ASCII terminals
located up to 1212 meters (3963 feet) from the system unit.
Note: If you use cables longer than 122 meters (399 feet), you must
provide surge protection. Also, do not install these cables outdoors.
Interrupt Level Switches
Use the following diagram to set the interrupt level on all the 4-port
adapters. Nonbuffered 4-Port Asynchronous adapters should be
grouped on one interrupt level, and Buffered 4-Port Asynchronous
Adapters should be grouped on another, preferrably higher, interrupt
level.
Interrupt Level 10 is recommended when using the 4-Port
Asynchronous Adapter together with the SCSI Adapter. If the 4-Port
Asynchronous Adapter is set for Interrupt Level 11, certain