Since most messages on a MSTP network are token passes a device that uses the token in 5 msec will consume much less bandwidth than one that takes 15msec. (A number of vendors relax this requirement to allow for other vendors implementations. A number of vendors have a range of pull up/down resistors installed and allow you to change the selection using software or jumpers. Some vendors may make chips which can drive more but they can't push the standard too far or it may present an electrical risk to the other devices. 2. Peter says: January 8, 2009, at 2:20 pm (Edit) The Standard imposes the limit on the number of unit loads presented by the device and does not limit the number of devices. I think you may be able to add a few more devices and still have a fully operational network depending on the load presented by the terminating resistors. When a device drives the line it sees the load presented by the other devices and the terminating (and biasing) resistors. This device and/or the pull up/down resistors of other devices are candidates for investigation. While handshaking is still with us, it is usually best to disable it in software (if possible) and/or "loop-back" the pairs of signals (RTS to CTS, DTR to DSR, etc.) While RS232 was rumored to be on the "way out" with the advent of many of the new communications standards, it is still alive and well today.

If a device floats out of the specified range then to other devices it will look like the floating devices isn't floating at all. It must not be allowed to float at any voltage level so devices have pull up/down resistors to pull the line to an allowable floating voltage. Buy two resistors of the same value. To use two wires ( as opposed to full duplex 4 wire) for RS485 each devices transmitter and receiver must be set to an idle state to release the line for others use. Line Drive On / OffTo use two wires for RS485 each devices transmitter and receiver must be set to an idle state to release the line for others use. The RS232 protocol provides for four handshaking signals called ready to send (RTS), clear to send (CTS), data set ready (DSR), and data terminal ready (DTR) to coordinate the transfer of information.

Rather, it relies on software handshaking via transmission of XON/XOFF characters to coordinate data transfer and ensure that information is not lost when one of the communicating parties is busy. The configuration and specifications of RS485 make it faster and extend the range of data transmission. RS485 was probably intended to be as flexible as possible. What happens if one device waits too long after sending its last bit before releasing the line it's possible that the other devices will miss some bits of data. The other devices will think that it is transmitting or receiving and thus blocking the line. If the vendor did a poor job, the default is to have the resistor active and they must be disabled unless they are the terminating devices on the network. In this case, cable connections must be made to Serial 1 at pins 5 and 6 of the 10-pin Serial Header or pins 7 and 8 on the 24-pin Field Header. There are many different types of serial communication protocols like I2C and SPI which can be easily implemented with Arduino and today we are going to look at another most commonly used protocol called RS485 which is very commonly used in high noise industrial environments to transfer the data over a long distance.

Just like it’s older brother RS232, RS485 is a form of serial communication. To enable the secondary serial port, turn dip switch 5 (labeled "2COM") on the QED Board ON. The Serial 2 port is dedicated to RS232 communications at up to 4800 baud. Finally, for master devices, the SPR1 and SPR0 bits determine the baud rate at which data is exchanged. Any required SPI output signals must be configured as outputs, either by calling InitSPI() or by setting the appropriate bits in the Port D data direction register DDRD. Once the data has been exchanged, a flag bit in the SPSR status register is set to indicate that the transfer is complete. The equipment located along a set of RS-485 wires are interchangeably called nodes, stations or devices. That decision set the limit at 32 unit loads (15kOhm each). The people who designed the standard made an arbitrary decision with practical implications How big a load can the line driver see without the signal being attenuated too much. The recommended arrangement of the wires is as a connected series of point-to-point nodes, a line or bus. If someone had to read just this article before touching a RS485 bus they would in for a world of hurt!

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