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Therefore, while not technically correct, it does make some sence to refer to a 4-wire RS485 network that would extend the number of nodes on a 4-wire network to 32 standard loads. On the other hand, DMX chose a nice round number at what was then the top of the range and added protections to avoid pick up of "ridiculous" frequencies, what is rs485 cable that we would now consider for actual data. Simply using the provided USB converter cable is not working for me right now with the Pi. Typing a carriage return at the terminal should now produce the familiar "ok" response via the Serial2 port. RS485 is another protocol supported by the primary serial port on the QVGA Controller. Henceforth, we shall consider Slave devices to be measuring instruments with serial communication, even if the cabling is similar for all Modbus devices. A Modbus RS485 connects a Master device to one or more Slave devices. The /SS (active-low slave select) is typically used to enable data transfers by slave devices when it is active low. The most common cable type is Cat 5e-UTP (unshielded twisted pair) which may work over shorter distances in less demanding applications with low EMI noise levels.

Other times you may write something and read it back to verify. What I want to be able to do is to communicate (or at least read from) the charge controller from a Raspberry Pi. Sometimes you can read from some register that has a known value. The value of each termination resistor should be equal to the cable impedance (typically, 120 ohms for twisted pairs). But often when that happens it's more likely to be referred to as RS422 as this standard strictly defines those 2 separate pairs. Unlike what happens in many energy distribution systems, the manner in which the devices are connected in parallel is important. Each device has a communication port with two terminals, which are indicated for the sake of convenience as A and B. In these two terminals the communication cable is connected so that all the devices that take part in the communication are connected in parallel. In some countries, inserting two cables into the same screw terminal is permitted.

For devices where you have to consider the devices at the far end of your cables to be unable to be modified, either hardware or software wise, you have to get clever regarding how you determine if the device is connected. No I have not been able to test this using the Windows software. The device ships with an Ethernet RS-485 communication interface as well as a RS-485 to USB cable, which is said to be able to communicate with a PC using proprietary Windows based software (and driver, I think). The maximum number of repeaters that should be serially connected is 3. A higher number introduces excessive delays in the communication system. Using a repeater, the main cable is divided into different segments, each of which can be up to 700 m in length and connect 32 devices (this number includes the repeaters). The maximum number of devices that can be connected to a main cable is 32, including the Master. The main cable must be no longer than 700 m!

Longer branches could cause signal reflections and generate disturbances and consequent errors in the reception of data. Moreover, if Serial2 is running full duplex at 4800 baud, any other interrupt service routine that takes longer than 100 µs is likely to cause a problem. The DMX standard is very strict: Each pair is unidirectional (like RS422), it uses longer mark and space times between datablocks than specified in computer Serial communication. The branches must be no longer than 1200 m! This distance does not include the branches (which must nevertheless be short). All the ‘A’ terminals must be connected together and all the ‘B’ terminals must be connected together respectively. They have Shielding Jacket over the insulation layer to protect against the Electromagnetic Interference and also each pair of wires is twisted together to prevent any current loop formation and thus much better protection against the noise. Thus in Table 11 5 , RTS1 is connected to CTS1, and DSR1 is connected to DTR1 onboard the QVGA Controller using zero ohm shorting resistors.