RS485 MODBUS Slow Response

O

Thread Starter

onionsjk

I am trying to establish two RS485 Modbus communications between Siemens (417-H CPU) and Yokogawa DCS (ALR121) which can take up to 2 connections. Siemens side 2 CPUs and Yokogawa side one card and each connection is by 2 wire. In fact Siemens side with identical logic and with different machine.

What I am witnessing is: if I test individual connection (one connection to ALR121), both communications are fine. Within 1-2 sec signal reach each other. Whereas when I make full connections (two connections to ALR121), there is 5~10 sec signal delay and even some signal get lost.

Each connection has same address scheme with different slave ID. Connection parameter used is parity=None, data bit=8, speed=19200. CPU loading of DCS side is lower than 40 %.

Can someone tell me about the maximum data one ALR121 card can take? Or something about cyclic update mode? What other parameter can I think of?
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You have not clarified but I'm guessing what you define as 'full connections' is 2 Modbus masters and 2 Modbus slaves on a single RS485 local network.

If this is the case then you may have two masters attempting to take control of the 485 network. The probable result is that both may detect data collisions, abort transmission then timeout a silent listening period awaiting the other to finish.
 
2 master connections are not allowed on a single RS-485 bus. You will get data collisions and what appears to be slow response time. We have a device which allows 2 master devices to talk to devices on the same RS-485 bus. Our TSH modules buffer requests from 2 masters so that only 1 is communicating at a time.
TSH-735

http://www.icpdas-usa.com/tsh_735.html
 
A quick scan of the ALR121 docs shows one needs 2 cards for 2 connections. So to allow 2 masters, or even 2 redundant connections to one master, you'll need Yokogawa cards.

- Lynn A Linse,
cradlepoint.com
 
Thank you all for sharing your opinions.

At first, I thought ALR121 card is one master for two connections but it is not logical. It should be one master for each connection and there will be two masters in ALR121 card. Anyway the below is the parameter error found:

=============================
Finally, solved.
It was error in configuration.
Following is working configuration.

- Parity [Even]
- Data bit [8]
- Stop bit [1]
- RS Control [ ]
- DR Check [ ]
- CD Check [ ]

=-- Communication Error Process --=
- Response Timeout [ 4] sec
- Number of Retry .. [ 1] msec
- Interval of Conn Retry [30] msec
- Inter-Character Timeout [10] msec ----> somehow this was [10000] msec


=--- RS,DR,CD ? --=
I think RS (Ready to send), DR (Data Ready)
CD (Carriage Detector) are not applicable for 2 wire
or even 4 wire. Can someone explain me the
mechanism if they are used in RS485 ?
As far as I know, RS,DR require hardwire cable in RS232C.


Regards

Jinki Jung
 
> =--- RS,DR,CD ? --=
> I think RS (Ready to send), DR (Data Ready) CD (Carriage Detector) are not
> applicable for 2 wire or even 4 wire. Can someone explain me the
>mechanism if
> they are used in RS485? As far as I know, RS,DR require hardwire cable in RS232C.

Correct - those signals are transmitted on additional RS-232 wires/pins which aren't present in 2 wire RS485 (or 4 wire, if using separate RX and TX pairs for a full-duplex connection).

FWIW, "CD" is "Carrier Detect". It's a modem control signal which indicates the modem is detecting the proper audio frequency, or frequencies, which indicates it is communicating properly with a remote modem. Also, "RS" is "Request To Send" (the device wishes to transmit data and expects to see "Clear To Send" prior to doing so) and "DR" is "Data Terminal Ready". Just so you know.
 
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