4 Channel Backplanes
The 4-channel backplanes are designed for stereo Channels. While mono or inline modules are usually connected to one DAW input and output per channel, stereo channels need 2 DAW sends and returns. When using many stereo channels that are used for stereo outputs and inputs of the DAW, the breakout shema of the 8-channel backplanes is not suited well. Both versions of the 4-channel backplane use 1 25-pin D-sub for the DAW return and another one for the DAW send; however, the 8 tracks on each D-sub are connected to four channels instead of 8. The tracks 1 and 2 a connected to the left and right inputs of the first channel slot, tracks 3 and 4 to left and right of the second channel slot, and so on.
Since stereo input modules don't have mic and line inputs, there are no additional XLR's. The TRS versions is shown above. The 2 insert I/O's per channel are available on 4 TRS jacks per slot. The image below shows the patch bay version with 2 additional 25-pin d-Subs for the inserts. One d-Sub is used for the 4 stereo insert outputs and the other one for 4 stereo insert inputs.