New answers tagged rs485
1
use pair 2 as YZ instead of ZY.
TIA 568 has pair two (3,6) coloured backwards to make the connector look prettier.
2
If both devices respond at approximately the same time then you’ll have a collision and can expect data errors. For RS485 you should make sure that only one device is transmitting at a time, so either poll the devices individually or add delays so that each device responds in its own ‘time slot’. At 9600 baud the data rate is about 1000 bytes per second, ...
3
It makes sense to distinguish between transmission and reception of data as they are quite different in this context.
Transmission is straightforward. If several bytes are ready to be transmitted, DMA can be configured for this specific length and the transmission can be started.
It is usually helpful to add a buffer. So if the code wants to transmit ...
0
The buffer size is constant, can't be variable, you define it in the declaration. The received or transmitted data length is not constant, meaning the buffer is several times larger than largest data packet. Depends on your architecture, maybe you can trigger an interrupt if new data don't arrive within few clock cycles. Assuming the transmitting device ...
1
The 667 mW number is the maximum allowable power dissipation for this device. This spec is probably based on the maximum die temperature and the thermal resistance of the package. That doesn't mean that it actually will consume that much power.
It looks like you need to allow for 1.5 mA at your maximum supply voltage, plus the power dissipated in the load.
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To overcome those problems, you should start to think about using isolated transceiver. Additionally you would need few RS485 repeaters, because a tiny 485 transceiver can't drive 100 devices at once.
By the way, you don't connect the PE directly on transceiver, it's just for the ESD protection potential. If integrated into IC then yes, you connect it to GND ...
1
You're probably not waiting until all the transmit bytes have been sent before changing the rs485 bus direction. Under Linux using a usb->rs485 converter takes care of this in hardware. When using the on chip serial ports, you'll probably need to read up on the serial driver and use the RTS to control the bus direction. Failing that, add a delay of a few ...
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the tricky part with your updated schematic would be getting the voltage on GPIO1 and GPIO2 to a voltage somewhere between A+0.6V and B+0.6V the absolute bus voltages in RS485 are poorly constrained. (could be anywhere +/- 12V)
maybe something like this instead:
simulate this circuit – Schematic created using CircuitLab
this uses a current source ...
1
The impedance should not vary with the length of the cable. It is a combination of the cable inductance with its capacitance:
From this article.
See also excellent answers here.
There are ICs with "Activatable" termination (such as LTM2881) but that is for connections that are not point to point.
Your proposed "solution" would just try ...
2
Question 1) Cable selection; UTP vs STP; Grounding:
Modbus standard requires shielded cable, so you must use STP.
Modbus standard also requires shield to be connected to protective ground, on one end only, so that there can not be any current flowing between devices via the cable shield. If there is a connector, cable shield must be connected to connector ...
1
You can use a dedicated TP cable, example: UNITRONIC Li2YCY (TP)
You may not do a star topology, rather you "stitch" devices together with a single cable.
should I use STP cables instead of UTP?
A TP cable has a rather thick screen, so no UTP should be used.
how do I ground the cable properly?
The shield has to be grounded at one point, only. ...
1
its bad juju to run comms in parallel to mains. You don't want to make a transformer.... shield or no shield. It's also called an electrostatic shield - not electromagnetic.
How to ground the cable - you really have only one ground point as the supply is via the cable.
Make sure you have protection for the cable on the 48V supply - you don't want a fault to ...
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It depends on the type of wires you are using, my personal experience is that UTP ethernet is fine, twisted pair helps to reduce EMI and therefore noise in control wires.
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