These modules may have either the UP/DOWN switch type or the ON/OFF slide switch type. Each type is illustrated below.
Switches
up 
down
Figure D-1 Rear View of ATL-CSU/DSU Link Module with UP/DOWN Switches
Figure D-2 View of ATL-CSU/DSU Link Module with Sliding ON/OFF Switches
When connecting two bridge/routers back-to-back with CSU/DSU link modules, a null-modem cable is required to crossover the pins on the links. Crossing over the pins allows two bridge/routers both configured as DTE interfaces to be connected together.
Switch number 1 determines whether the ATL-CSU/DSU link module will generate clocks or receive clocks. When switch 1 is down (on), the normal position, the module receives clock signals from the connected network. When switch 1 is up (up), the module will generate clocks. When a pair of routers are connected back-to-back with CSU/DSU link modules one module must be set to generate clocks and one module must be set to receive clocks.
On 64 Kbps units only, switch number 3 determines the mode of the ATL-CSU/DSU. When switch 3 is down (on), the ATL-CSU/DSU is in DDS (Digital Data Service) mode for normal connection to the 64 Kbps digital service. When switch 3 is up (off), the ATL-CSU/DSU is in LDM (Limited Distance Modem) mode for back-to-back connection with a null-modem cable.
On 56 Kbps units, the position of switch 3 is not a factor for back-to-back connection with a null- modem cable. Switch 1 must still be set as noted above.
A DSU/CSU crossover cable would be constructed as follows: 1 --> 7 2 --> 8 7 --> 1 8 --> 2
