Applies To:
  • CitectSCADA x.x

Summary:
Question: What determines when an I/O unit goes offline? 

Solution:
When using Citect in a redundancy situation, an I/O device must tell Citect when it cannot obtain data from the device by going offline. Citect will only switch to an standby I/O device if the current I/O device goes offline. An I/O device indicates to Citect that it has gone offline by sending Citect a generic error 12 (decimal).

If an I/O device is no longer functioning and is responding with errors to Citect requests, Citect will still consider the I/O device as online if the non functioning I/O device never reply with generic error 12. This may cause the redundancy to fail to operate correctly if the protocol driver does not recognise a particular error as fatal. If you find a fatal error which does not cause a redundancy changeover check to make sure you have the latest driver or check with Citect support or the driver developer.

For most protocols the I/O device will be put offline if it gets a certain number of consecutive timeout errors. The number of consecutive timeouts depends on the number of retry allowed. It is recommended to protocol driver writers that the I/O device must be put offline if the number of consecutive timeouts exceeds (retry + 1) * (retry + 1). The following protocols follows this rule:

ABTCP, IZUMI, BAILEY, TISERIAL, OPTOMUX, MELSEC, MODBUS, OMRON, TICIM, TIDIRECT, RELIANCE, GENIUS, SIEMENS, TXMITTON, TRIODIAG, TOSHIBA, MENTOR, TOSDIC, OMFINS, HITACHI, HR2400, KE, FUJI, SBUS, CCM and COMLI.

For some protocols the I/O unit will be put offline if it gets a specific driver error. By trial and error each protocol must be coded to recognise these special driver error codes. The following protocols recognise special driver error codes that will put the I/O unit offline. This list is not complete and will be updated as they are reported by users.

MBPLUS 0x215 0x212 0x211 0x20a 0x208 (all added on 1st June 1995)
0x203 (added on 19th July 1995)
ABKT
ABEI 0x10100 0x10200 0x10400 0x20036 0x40041
KT 0x10100 0x10200
0x10400 (When DH+ cable is disconnected on PLC-3, added on 11th September 1996)
GENIUS 0x10b
0x20 (Hot standby variable failed, added on 13th November 1996)
ICN 0x18e 0x18f
TAYLOR 0x18e 0x18f
BAILEY 0x109 0x112
SQUARED 0x10011
0x1000b (added 16th October 1996)
TIWAY
TISERIAL 0x0488
RNET 0x2c0
OMFINS 0x19005 (added 4th January 1996)
KE 0x10200

For some protocols the driver error will be considered to be so serious that the channel that the I/O unit is attached must be put offline to allow Citect to re initialise the whole channel. The following protocols recognise special driver error codes that will put the channel that the I/O unit is attached to offline. This list is not complete and will be update as they are reported by users.

TIWAY
TISERIAL 0x100 0x0401 0x040b 0x040d 0x140b 0x4404 0x0484
ABTCP 0x10200 0x10400 0x10500
0x50200 (added on 29th April 1996)
0x20 (Hot standby variable failed, added on 13th November 1996)
ABKT
ABEI 0x4002E 0x40036 0x40038 0x40039
GENIUS 0x101 0x102
 

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