Protocol for RS485, Differences between 8433 & 8431

[GaryZ21]

1) Is there a difference between the PCI 8433/4 and PCI 8431/8 in terms of isolation, bias resistors?  Are the bias resistors added on the 8 port card?  If not, what is the reason for leaving them out?


2) What does a "break" error message mean from "GetComStat()"?  If the com status report a  break or any error, does the driver  zero out the received byte information?


3) Is there a "Stop" bit check from the "GetComStat()? or any other functions"? 



4) What is the threshold for a logic low and what's the threshold for a logic high in the RS485 communication?


5) What's the protocol for reading the start bit?  Ex: if the comm is set for 8bit, 1 Parity, 2 Stop.   Will the Rx or the driver read every twelve bit and validate the start bit based on the first bit of every twelve bits, from the received data? 

[Ebele_Obijiofor]

Hello,

We don't include termination resistors with our 8 and 16 port boards. 
A break error occurs when the connection is broken, and it is detected when the transmission line does not send the stop bit.
The GetComStat() function does check stop bits; You can find the manual here, and there's information about the function on page 5-28.
Here's some information for logic high and low: " The [485] standard specifies a differential form of signalling. The difference between the wires’ voltages is what conveys the data. One polarity of voltage indicates a logic 1 level, the reverse polarity indicates logic 0. The difference of potential must be at least 0.2 volts for valid operation, but any applied voltages between +12 V and -7 volts will allow correct operation of the receiver"

Hope this helps. Thanks

Regards

[JasonSmith]

Hi Gary,

The PCI-8433 provides port-to-port and port-to-computer isolation, while the PCI-8431 does not. 

The eight and sixteen port cards do not include sockets for bias resistors due to the large amount of space they would consume on the board, since four sockets are needed for each port. There are surface mount pads on the eight port board which can be used for bias resistors, although soldering components to the board would void the warranty.

Jason S.
National Instruments