Can I connect a single fp-1001 to two fp-1000's each on seperate cpu's?

[PMW@RPT]

Is it possible to poll a single FP1001 from two computers using an FP1000 on each of the computers?

[Aaron_G]

No. It is not possible. The FP-1000's will transmit any data from the RS-232 port onto the RS-485 port. If you wire two FP-1000's together, then you risk both of them attempting to drive the transmit line at the same time. RS-485 is designed for a single master, it is not designed to allow multiple devices to drive the line simultaneously. You may or may not cause any damage doing it, but you will screw up the data transmission if both computers are trying to talk at the same time.
On the other hand, you may be able to get a Serial Port Smart Switch, that multiplexes 2 RS-232 ports to 1 (or vice versa). This is a RS-232 box that automatically switches between active ports. In this case you could use both computers to talk to an FP-1000. However, depending upon
how the switch works, a simultaneous poll may result in one computer returning a time out error or may result in both computers having corrupted data.

Regards,
Aaron

[Rainer Ehrt]

Hi Aaron,
sorry, you're wrong.
Aaron G wrote:

> No. It is not possible. The FP-1000's will transmit any data from the
> RS-232 port onto the RS-485 port.

Have a look at the description. This is as its written in the EIA Catalog
(Electronics Industries Alliance Standards and Engineering Publications).
This Publication is for free: http://www.tiaonline.org
The complete Standard papers can be bought for the price at the end of the
next paragraph.

###
TIA/EIA-485-A
Electrical Characteristics of Generators and Receivers for Use in Balanced
Digital Multipoint Systems.
(ANSI/TIA/EIA-485-A-98)
This Standard specifies the electrical characteristics of generators and
receivers that may be employed when specified for the interchange of
binary signals in multipoint interconnection of digital equipment. When
implemented with the guidelines of this standard, MULTIPLE GENERATORS AND
RECEIVERS may be attached to a common interconnecting cable.
Product Code 3 Mar, 1998 COMMITEE: TR-30.2
$49.00
####


> If you wire two FP-1000's together,
> then you risk both of them attempting to drive the transmit line at
> the same time. RS-485 is designed for a single master, it is not
> designed to allow multiple devices to drive the line simultaneously.
>

No, ist definately for multimaster and its half duplex and needs only two
lines.
RS-422 is what you describe, which is full duplex and needs 4 lines.


> You may or may not cause any damage doing it, but you will screw up
> the data transmission if both computers are trying to talk at the same
> time.

IF NI says, their RS485/422 hardware can do RS-485, then damages must be
covered by warranty.

So lets get thing working
Whats wrong:

NI sometimes write RS-485, but infact means RS-422.

Sample: Measurement and Automation Catalog jan. 2002, page 505 ( Or have
take a look in the contents of the catalog at "Distributed I/O",
"Field-Point".
There is a picture showing connections between a computer- RS-232-
FP-1000-RS-485- 2 FP- modules.
At this place things are wrong, because cabeling is 4- wire and true
RS-485 is 2 wire!

Whats correct:

Othertimes NI knows and also writes the correct pinout of connectors and
cabeling of RS-485:

Sample: NI Serial Device User Manual of my ENET-485/4, Appendix D, Serial
Port Information.
At the chapter RS-485, They write: "Unlike the RS-422 standard, RS-485
adresses the issue of using multiple transmitters on the same line. RS-485
can also withstand multiple drivers driving conflicting signals at the
same time."
Other NI- Serial Port device manuals may contain similar texts.

So here's the solution:
If NI says FP-modules can handle RS-485 and there are only 4 connection
cables for the network connection, then one has to bridge TX+ with RX+ and
TX- with RX-. In this way you just have 2 network wires left. When doing
this, RS- Network devices may handle this situation automaticly, or they
have to be configurated.
For my ENET 485/4, this means:
4-wire RS-422: Serial Port config: Tranceiver mode = 4 wire
2-wire RS-485: Serial Port config: Tranceiver mode = 2 wire TxRdy Auto

So how should involved RS-485 devices behave:
By default, RS-485 all active devices of a network are in Read- mode, so
there are no conflicts. If one device wants to write to the bus, ist
switches its tranceiver to write mode, writes the data to the bus and goes
back to read mode again. The tranceiver switching time from read to write,
or back from write to read, should take 400 �seconds each. People working
with labview have to insert 1ms delay if they want ro read data from the
serial port, after writing.
See National Semiconductor application notes:
http://www.national.com/pf/DS/DS36C278.html#Application Notes
Some timing info, look AN-702 at
http://www.national.com/appinfo/interface/0,1801,121,00.html

My National Instruments ENET-485/4 (Ethernet to 4-port RS-232/422/485)
needs 20ms (= 50 X 400�s) though. The Ni- Support couldn't help me in this
case yet. I don't know how NI-PCI-Serial Port cards behave, but I used a
dozen of RS-232/485 Converter of Adlinks NuDAM series: ND-6520.
http://www.adlink.com.tw/products/RemoteData/ND-6520102130.htm
They work perfect, connected right to the PCs- serial port. They can
handle RS-485 and 422.

In the future, if NI can't get my ENET-485/4 do work with correct timing,
I'll try MOXA http://www.moxapartner.com/ .

Well, this was lots of technical RS-xxx info.

Rainer Ehrt

[Aaron_G]

Rainer,

Thanks for the links and detailed info. I will amend and clarify my previous answer in the following ways.

1) RS-485 does allow is multi-master/multi-slave configurations. Some/most RS-485 transceivers have built-on protection against bus contention (two devices driving the line simultaneously).
2)The FP-1001 modules do support both 4 wire and 2 wire modes with no difficulty or need for any additional configuration changes beyond the wiring Tx+ to Rx+ and Tx- to Rx-.
3) The RS-485 ports on the modules will not be damaged if two devices try to drive it at once. They do have protection against damage from bus contention problems.
4) However, despite #3 above, if both computers try to access the FP-1001 at the same time, both are l
ikely to get garbled/garbage data. In all likelihood, the data will be rejected by the FieldPoint server as the checksums will not match.
5) The RS-485 repeaters on the FP-1000 do not support 2 wire mode.

Regards,
Aaron

[PMW@RPT]

Thanks for all of the help. We will only need the 2nd unit temporarily. During this period the data is not critical, and no control is taking place. We will give it a try and see what happens.