PCB Review Agenda 20-01-2012
The DEM files of the project 02446 can temporally be found here.
In the PCB Review to be held the Friday 20th January the main points to be discussed are:
- Schematics
- Isolation: options and compatibility with legacy boards.
- Rear Transition Module pinouts and compatibility with CONV-TTL-RS485: adding RTM passive[?] IDs
- PCB received from DEM
- Issues found: like 3V3_IC7 power plane, small[?] 1V2 plane for FPGA, FOD060L replacement, adding buffers for the RTM passive and pins in FPGA.
Schematics
Isolation
Different isolation connections are found at CERN. However, there's not an strong agreement of how to specifically do this for the case of either the Blocking boards or the next RS-485 ones. A couple of good references for gaining insight are "Goods for grounding"[1], which offers a system point of view for solving grounding issues, and "Noise Reduction Techniques in Electronics Systems"[2] that offers a quick and straightforward technical approach for cable grounding.
The isolation problem found in the schematics lies in the ground floating state of both the driver and the receiver, which as pointed out in [1], can yield into shock and spark hazard ([1] section 4.2.2.1). Thus, an acceptable schema for the Blocking board can be as follows:
- Driver will be isolated (via couple inductors)
- Receiver must be grounded, so as BLO- input pin (cable shield).
[2] explains that for low frequencies, like the ones expected for the Blocking case (< 10MHz), breaking the ground loop by only grounding at one end is effective.
So as to be compatible with legacy boards, the receiver should be grounded and changes should be done in the Schematics and PCB.
Another issue found is the Fairchild's FOD060L should be replaced due to difficulties to be supplied immediately. However, pin-to-pin compatible replacements from Fairchild (HCPL-0601, HCPL-2601, 6N136, 6N137) or Avago (HCPL-2611) are widely available.
Rear Transition Module
Identification*: A proposal can be found in this wiki page. Pull-ups and buffers must be added to the design and P2 pinouts should be done.
P2 pinout modification*: so as to build a good generic RTM motherboard a compatible pinout must be set up between CONV-TTL-BLO and CONV-TTL-RS485 projects. A pinout proposal can be found here.
Layout
Main issues found:
- Plane 3V3_IC7 > Plane 3V3
- Possibly 1V2 can be bigger layouted
[1] "Grounds for Grounding", Elya B. Joffe
[2] "Noise Reduction Techiniques in Electronic Systems", Henry W. Ott
Carlos Gil Soriano - 19 January 2012