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Reset (4 V option only)
Table 9: /Reset Parameters
| Absolute Maximum | -15 V to + 20 V |
| Logic High | > 2.3 V |
| Logic Low | < 1.0 V |
| Protection | 2 kW series resistor, 6.8 V TVS diode |
| Termination | 2 kW to +3.3 V |
The /Reset signal is used in the factory during the bootloader programming operation. In flight it should be left unconnected, with the wire snipped as close to the connector as possible. The internal pull-up resistor will keep the star tracker running normally.
2.7. Command [A|B] (RS485)
Table 10: Command Parameters (RS485)
| Absolute Maximum | -11 V to +15 V, each signal, WRT Power Ground |
| ESD rating | ±15 kV (Human-body model) |
| Polarity | B > A in Mark (Idle, “1”) state A > B in Space (On, “0”) state |
| Input Resistance | > 96 kW each signal to Power Ground |
| Input Differential Threshold | ± 0.2 V max |
The RS-485 pair carrying commands from the spacecraft to the star tracker is called Command. Pay very careful attention to the polarity designation. The formal standard of EIA/TIA-485 is used. Some devices/chips use incorrect A/B designations.
The Command signals are inputs to the star tracker. No line termination is used.
Star trackers will only interpret commands that are addressed to them. Thus, two star trackers with different NSP addresses can share a common Command link.
2.8. Telemetry [A|B] (RS485)
Table 11: Telemetry Parameters (RS485)
| Absolute Maximum | -11 V to +15 V, each signal, WRT Power Ground |
| ESD rating | ±15 kV (Human-body model) |
| Polarity | B > A in Mark (Idle) state A > B in Space (ON) state |
| Differential Output Voltage | > 2 V into 50 W termination. > 1.5 V into 27 W termination. |
| Short-circuit Output Current | 300 mA max |
| Three-State Output Current | ± 10mA max |
| Output Impedance | 100 W @ 100 MHz, common-mode |
The RS-485 pair carrying telemetry from the star tracker to the spacecraft is called Telemetry. Pay very careful attention to the polarity designation. The formal standard of EIA/TIA-485 is used. Some devices/chips use incorrect A/B designations.
The Telemetry signals are outputs from the star tracker. When there is no data to transmit the star tracker will three-state its outputs to allow another unit to drive the bus. Thus two star trackers can share a common Telemetry link.
The Telemetry line driver IC is equipped with an overtemperature shutdown feature which will turn it off if it overheats: for example, if the bus is short-circuited for an extended period.
2.9. RS-485-[0|1] [A|B]
| Absolute Maximum | -70 V to +70 V, each signal, WRT Power Ground |
| ESD rating | ±16 kV (Human-body model) |
| Polarity | B > A in Mark (Idle) state A > B in Space (ON) state |
| Differential Output Voltage | > 1 V into 54 W termination. |
| Short-circuit Output Current | 200 mA max |
| Input Resistance | > 96 kW each signal to Power Ground |
| Input Differential Threshold | -0.18 V to -0.035 V |
| Three-State Output Current | ± 100 mA max |
The 28 V option electronics feature one or two bidirectional RS485 pairs. Pay very careful attention to the polarity designation. The formal standard of EIA/TIA-485 is used. Some devices/chips use incorrect A/B designations.
Command (ASYNC)
Table 12: Command Parameters (ASYNC)
| Absolute Maximum | -15 V to +20 V, WRT Power Ground |
| Input Voltage WRT Power Ground | > 2.2 V in Mark (Idle, “1”) state < 0.7 V in Space (On, “0”) state |
| Input Resistance | ~100 kW to +3.3 V |
The low-voltage CMOS signal carrying commands from the spacecraft to the star tracker is called Command. The Command signal is an input to the star tracker. No line termination is used, but a small current source biases the signal high if it is left floating.
Star trackers will only interpret commands that are addressed to them. Thus, two star trackers with different NSP addresses can share a common Command link. The Command signal is available on two pins to facilitate daisy-chain wire harnesses.
Telemetry (ASYNC)
Table 13: Telemetry Parameters (ASYNC)
| Absolute Maximum | -15 V to +20 V, WRT Power Ground |
| Output Voltage | +3.3 V in Mark (Idle, “1”) state, 4.7 kW series 0.0 V in Space (On, “0”) state, 4.7 kW series +3.3 V in high-Z state, ~100 kW series |
| Short-circuit Output Current | 300 mA max |
| Three-State Output Current | ± 10mA max |
| Output Impedance | 100 W @ 100 MHz, common-mode |
The low-voltage CMOS signal carrying telemetry from the star tracker to the spacecraft is called Telemetry. The Telemetry signal is an output from the star tracker. When there is no data to transmit the star tracker will three-state its outputs to allow another unit to drive the bus. Thus two star trackers can share a common Telemetry link.
The telemetry output has a series resistor to limit the current. It also has a small current source to bias the signal high in high-Z mode. Care must be taken to ensure that the pull-up is sufficient, but also that the signal has enough drive to overcome external pull-up devices in the Space state.
2.12. CAN[_L|_H] (CAN)
Table 14: CAN Parameters
| Absolute Maximum | -80 V to +80 V, each signal, WRT Power Ground (4 V option) -36 V to +36 V, each signal, WRT Power Ground (28 V option) (But do not exceed 0.25 W continuous into termination resistor if installed) |
| Input Common Mode Range | -7 V to +12 V |
| Dominant Input Voltage | VCANH – VCANL = 0.9 V to 3.3 V |
| Recessive Input Voltage | VCANH – VCANL = -1.0 V to +0.5 V |
| Input Hysteresis | 0.15 V typ (4 V) 0.1 V typ (28 V) |
| Dominant Output Voltage | VCANH – VCANL > 1.5 V into 45 Ω load (4V) VCANH – VCANL > 2.3 V into 60 Ω load (28V) |
| Slew Rate | Configurable – Contact factory |
The CAN_L and CAN_H signals comprise a differential pair used for both input and output. In the 4 V option the signals are available on two pins each to facilitate daisy-chain wire harnesses.
Date: 2015-12-17; view: 757
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