The receiver can monitor the wires and indicate a fault if a 0 is detected on the wires for an extended period of time. Once again utilizing the 74HC2G34 but this time as a high-to-low level translator (U402). Typically I'd include ESD protection on every output pin connected to the external connectors (J401, J402), but in this case the 74HC2G34 has built-in ESD protection. It's often to see these type of protection devices omitted from hobbyist designs, RS485 standard but for hardware like this these protections are essential. To solve the "data collision" problem often present in multi-drop networks hardware units (converters, repeaters, micro-processor controls) can be constructed to remain in a receive mode until they are ready to transmit data. RS485 is a hardware architecture that specifies only the electrical characteristics of transmitters and receivers, without endorsing any particular transmission protocol. The specification allows for data transmission from one transmitter to one receiver at relatively slow data rates (up to 20K bits/second) and short distances (up to 50Ft. @ the maximum data rate).
The legends and folklore, not to mention the flat out wrong information that has grown around the line length and data rate limits inherent in RS-485 are truly astounding. If you have to know that it will work before you pull the cable, then get the required length of the cable and move the two devices next to each other, connect them and see if they can talk. These buffers have clamping diodes. This is useful for firmware updates and such when it's less practical to have the full 24 V power on. I've add some length/skew compensation although frankly with USB Full Speed it's hardly required4. RS485 main advantages as compared to other serial communication are tolerance to electrical noise, lengthy cable runs, multiple slaves in one connection, and fast data transmission speed. Supports Third-party Data Loggers and IoT Platforms: Besides SenseCAP Data Logger, developers could connect to other data loggers that support MODBUS RS485 protocol and third-party IoT platforms. Connect the A and B to another one’s A and B of the RS485 port separately. The Laureate P/N LMOD interface board with dual RJ45 jacks allows multiple Laureate meters, counters, timers and remote displays to be connected to the same RS485 line.
R1001 is a pull-down resistor to make sure the MOSFET is off while the board is powering up. Make sure the switched pad of the MOSFET has thermal vias and plenty of copper area to dissipate heat. Keep the flyback diode (D1002) close to the connector to reduce the loop area for the flyback current. RS-485 does not specify any protocol, addressing scheme, or connector. The USB connector (J407) is a USB B jack because of its robustness. Starfish uses USB to connect the host machine and receive commands. Starfish needs to communicate with the outside world. Starfish uses a set of two pumps, valves, and sensors to control the vacuum system used to let the two nozzles pick up small components. This is enough to drive a small power MOSFET. Another relatively straightforward circuit consisting of a small MIC5317-3.3 linear regulator (U202). Since the drop from 5 V to 3.3 V and the current demands are rather small, a simple linear regulator works well here.
I chose the BUK9Y40-55B, but there are lots of MOSFETs that'll work here - they need to be have more than 24 V of drain-source voltage (VDS), and more than 1 A of drain current (ID) with gate voltage (VGS) of 5 V. The BUK9Y40-55B has a VDS of 55 V and an ID of 26 A at 5 V VGS - more than enough! Standards have been developed to insure compatibility between units provided by different manufacturers, and to allow for reasonable success in transferring data over specified distances and/or data rates. Yet few of these engineers have ever seen the standard, let alone read it. These valves are used to control the vacuum on the pick up head's nozzle - when the valve is open the nozzle can pick up parts using the vacuum, and when closed the nozzle is vented to atmospheric pressure so that it can let go of the parts. The big trick with using MOSFETs for load switching is that they really want significant voltage on their gate in order to completely open them up- 3.3 V GPIO doesn't really cut it. If you're interested in an open source pick and place, definitely check out the Lumen project.