(Related: https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=41418)
Relevant DCC: D1600298.
According to the circuit diagram (first attachment from V3 of the above DCC entry), the control voltage ("Switch") and the readback voltage ("Switch Readback") are always opposite, i.e. when control=high readback=low and vice versa. This was not the case for BOOST stages as measured at the rack. High/low is inverted in readback and "high" is only 3V. When I disconnected the readback cable from the chassis, 3V changed to 6V.
| DB9 pin |
Binary control cable 1 (V) |
Binary readback cable 1 (V) |
| 1 (DZ pole) | 15/0.7 | 0/15 |
| 2 (SERVO Close) | 15/0.7 | 0/15 |
| 3 (BOOST1) | 15/0.8 | 3/0 |
| 4 (BOOST3) | 15/0.8 | 3/0 |
| 6 (EXC) | 15/0.8 | 0/15 |
| 7 (PD select) | 15/0.8 | 0/15 |
| 8 (BOOST2) | 15/0.8 | 3/0 |
| 9 (OUTPUT On) | 15/0.8 | 0/15 |
I opened a spare chassis and found that the wiring for boost logic is indeed different. Second attachment shows some LEGO-type mid-air jumper job (yellow circles) as well as pins 2 and 3 short-circuited (red) for the Max4659 connected to the readback of BOOST binary inputs. Third attachment shows the same thing in a human-friendly form, but I don't know if pin 8 was completely cutt off from GND (or pin 2 from +15V).
Is this intentional? At least this explains the readback logic.
I still don't know exactly why High voltage reads 3V (readback cable connected) or 6V (disconnected) at the chassis. Voltage pulling sounds a bit too much to me.
The boost readbacks also go to the transimp boards where there are connected to an LED with 1k to ground.
As Daniel points out D1600193 (trans impedance board) has an LED there for each boost stage readback. Since boosts are designed as normally ON (i.e. HIGH=ON) in V3, these LEDs turn on when boosts are ON.
LED is ~1V, then 1k here and 2k in D1600298, so the voltage without the cable should be ~1V+(15-1)/3 ~5.7V ~6V. Makes sense.
When cable is connected it drops down to 3V, i.e. 2mA into 1k plus ~1V for LED in the trans impedance board. At the same time the current in 2k in D1600298 should be 12/2k=6mA. That means that the input of BIO is drawing (6-2)mA~4mA. Plausible.
PSLISS model was changed (work permit 7546, also see attached). It seems like the board switching is working as intended.
A note for myself. Green/yellow means logic inversion in the frontend/hardware. On the control side, I inverted in FE whenever the switch is normally ON in hardware (poles and boosts). On the readback side, there's no inversion in FE for boosts as they have the readback inversion in hardware.
| When you set this EPICS Channel to [0, 1] | FE output bit is set to | Control voltage is set to | Hardware is set to | Readback voltage is set to | Readback bit in FE is set to | _MON readback in EPICS is set to |
| PSL-ISS_SECONDLOOP_AC_COUPLING_POLE | [1, 0] | [L, H] | [OFF, ON] | [H, L] | [1, 0] | [0, 1] |
| PSL-ISS_SECONDLOOP_EXC_SWITCH | [0, 1] | [H, L] | [OFF, ON] | [L, H] | [0, 1] | [0, 1] |
| PSL-ISS_SECONDLOOP_ENABLE | [0, 1] | [H, L] | [Open, Close] | [L, H] | [0, 1] | [0, 1] |
| PSL-ISS_SECONDLOOP_PD_SELECT | [0, 1] | [H, L] | [PD14, PD58] | [L, H] | [0, 1] | [0, 1] |
| PSL-ISS_SECONDLOOP_BOOST_1 | [1, 0] | [L, H] | [OFF, ON] | [L, H] | [0, 1] | [0, 1] |
| PSL-ISS_SECONDLOOP_BOOST_2 | [1, 0] | [L, H] | [OFF, ON] | [L, H] | [0, 1] | [0, 1] |
| PSL-ISS_SECONDLOOP_BOOST_3 | [1, 0] | [L, H] | [OFF, ON] | [L, H] | [0, 1] | [0, 1] |
| PSL-ISS_SECONDLOOP_OUTPUT_SWITCH | [0, 1] | [H, L] | [Open, Close] | [L, H] | [0, 1] | [0, 1] |
| PSL-ISS_THIRDLOOP_POLE | [1, 0] | [L, H] | [OFF, ON] | [H, L] | [1, 0] | [0, 1] |
Also note for myself. Contec DIO-6464T2-PCI BIO card has negative logic (high=0, low=1) and their input pins are internally pulled up by VCC via 10k. aLIGO binary input chassis (e.g. D1001036) feeds the readback voltage of [L,H] into PSS12021 current source, thus [not-driving,driving] LED of opto-coupler, which in turn [cuts,pulls] the current drawn from BIO card input pin, thus setting the readback bits [0,1].
