For the Fibre-optic communications there are some conventions. The fibres which send commands to the module are called TX for "transmitter", also called TTC sometimes. The fibres which receive back data from the modules are called RX for "receiver."
Each module has 1 TX signal sent to it and each side on the module sends back 1 RX signal, so that each module has 2 RX signals coming back.
A fibre bundle (called a fibre, from now on) has 12 individual fibres in it, so for example, could carry RX signals from half a row, or readout half a row.
The fibres are attached to the BOC (Back of Crate) in the control room. Each BOC card has connections for 4 TX fibres and 8 RX fibres, which means that a card can control and readout 8 half-rows. Each BOC card has a corresponding ROD (Readout Driver) card on the front side of the crate. The ROD crate has a computer on it which controls the full crate as well as a TIM (Timing Interface Module) card which has to be in slot 13 (the middle slot in the crate), due to timing constraints.
In the control room, in the bottom of the BOC, is where all the fibres are located. They have been allocated according to their connections downstairs. In general, for
TX RX Sector or North Table : 1-16 1-32 South Table: Z- Barrel 3: 17-32 33-64 for additions (ie. Barrel6) 33-44 65-88 Z+ Barrel 3: 45-60 89-120 for additions (ie. Barrel6) 61-72 121-144
Note that Barrel 3 has 32 rows, corresponding to 32 TX fibres and 64 fibres.