Each silicon layer is read out by a high density interface (HDI), which is a printed circuit board with 25 front-end chips (GTFE64), two controller chips (GTRC), and passive components mounted. Two HDI's are attached on two sides of a tray, one on a side to read out a layer on the top side of the tray and the other on the opposite side to read out one on the bottom. The top-most and the bottom-most tray has only one HDI attached, since detectors are mounted only on one side of the tray. There are 32 HDI's in total with 64 controller chips and 800 front-end chips.
Each front-end chip reads out 64 strips, which covers 1600 strips with 25 chips on an HDI. Hit information of the 1600 strips is recorded for every trigger and kept in 8-deep data buffers of the front-end chips until it is read out. The data is read out by one of the controller chips. During readout, a front-end chip sends out the data to a neighboring chip, either on the right or on the left, and the neighbor passes it to the next neighbor. Readout direction can be set by command to individual front-end chip. The hit data is transfered in this manner all the way through the HDI until the last chip passes it to one of the controller chips.
Two controller chips on an HDI control all front-end chips on the HDI. One chip locates on the left side of an HDI and reads out left-going data, while the other on the right and reads out right-going data. Both of the controller chips can read out either the entire side of the tray or a part of it. Arbitrary number of front-end chips can be assigned to be read out from the left side or the right side of an HDI.
A controller chip is also an interface to a tower electronics module (TEM). It receives commands from a TEM board and sends out hit information to a TEM board after it reads out front-end chips and converts data format. These communication are made through a Kapton flexible cable connecting HDI's to a TEM board through a repeater board which buffers weak signals from HDI's. There are 8 HDI's on each of four sides of the tracker. A set of 8 HDI's is daisy-chained and connected to a TEM board with a Kapton cable. The left side controller chip and the right side one are connected independently, there are 8 cables connected to the TEM board in total.