SCADA Communications
P74x/EN SC/Na7
MiCOM P741, P742, P743
(SC) 13-5
SC
1.2.1.3 Bus connections & topologies
The EIA(RS)485 standard requires that each device be directly connected to the physical
cable that is the communications bus. Stubs and tees are expressly forbidden, as are star
topologies. Loop bus topologies are not part of the EIA(RS)485 standard and are forbidden
by it.
Two-core screened cable is recommended. The specification of the cable will be dependent
on the application, although a multi-strand 0.5mm
2
per core is normally adequate. Total
cable length must not exceed 1000m. The screen must be continuous and connected to
ground at one end, normally at the master connection point; it is important to avoid
circulating currents, especially when the cable runs between buildings, for both safety and
noise reasons.
This product does not provide a signal ground connection. If a signal ground connection is
present in the bus cable then it must be ignored, although it must have continuity for the
benefit of other devices connected to the bus. At no stage must the signal ground be
connected to the cables screen or to the product’s chassis. This is for both safety and noise
reasons.
1.2.1.4 Biasing
It may also be necessary to bias the signal wires to prevent jabber. Jabber occurs when the
signal level has an indeterminate state because the bus is not being actively driven. This
can occur when all the slaves are in receive mode and the master is slow to turn from
receive mode to transmit mode. This may be because the master purposefully waits in
receive mode, or even in a high impedance state, until it has something to transmit. Jabber
causes the receiving device(s) to miss the first bits of the first character in the packet, which
results in the slave rejecting the message and consequentially not responding. Symptoms of
this are poor response times (due to retries), increasing message error counters, erratic
communications, and even a complete failure to communicate.
Biasing requires that the signal lines be weakly pulled to a defined voltage level of about 1V.
There should only be one bias point on the bus, which is best situated at the master
connection point. The DC source used for the bias must be clean; otherwise noise will be
injected. Note that some devices may (optionally) be able to provide the bus bias, in which
case external components will not be required.
FIGURE 1: EIA(RS)485 BUS CONNECTION ARRANGEMENTS
It is possible to use the products field voltage output (48V DC) to bias the bus using values
of 2.2kΩ (½W) as bias resistors instead of the 180Ω resistors shown in the above diagram.
Summary of Contents for P741
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Page 6: ...P74x EN IT Na7 Introduction MiCOM P741 P742 P743 ...
Page 8: ...P74x EN IT Na7 Introduction IT 1 2 MiCOM P741 P742 P743 IT ...
Page 18: ...P74x EN TD Na7 Technical Data MiCOM P741 P742 P743 ...
Page 30: ...P74x EN TD Na7 Technical Data TD 2 14 MiCOM P741 P742 P743 TD ...
Page 32: ...P74x EN GS Na7 Getting Started MiCOM P741 P742 P743 ...
Page 70: ...P74x EN ST Na7 Getting Started MiCOM P741 P742 P743 ...
Page 72: ...P74x EN ST Na7 Settings ST 4 2 MiCOM P741 P742 P743 ST ...
Page 116: ...P74x EN OP Na7 Operation MiCOM P741 P742 P743 ...
Page 120: ...P74x EN OP Na7 Operation OP 5 4 MiCOM P741 P742 P743 OP ...
Page 136: ...P74x EN OP Na7 Operation OP 5 20 MiCOM P741 P742 P743 OP FIGURE 8 CB FAIL LOGIC ...
Page 166: ...P74x EN AP Na7 Application Notes MiCOM P741 P742 P743 ...
Page 234: ...P74x EN PL Na7 Programmable Logic MiCOM P741 P742 P743 ...
Page 290: ...P74x EN PL Na7 Programmable Logic PL 7 56 MiCOM P741 P742 P743 PL ...
Page 291: ...Programmable Logic P74x EN PL Na7 MiCOM P741 P742 P743 PL 7 57 PL ...
Page 292: ...P74x EN PL Na7 Programmable Logic PL 7 58 MiCOM P741 P742 P743 PL ...
Page 294: ...P74x EN PL Na7 Programmable Logic PL 7 60 MiCOM P741 P742 P743 PL ...
Page 295: ...Programmable Logic P74x EN PL Na7 MiCOM P741 P742 P743 PL 7 61 PL ...
Page 296: ...P74x EN PL Na7 Programmable Logic PL 7 62 MiCOM P741 P742 P743 PL ...
Page 298: ...P74x EN PL Na7 Programmable Logic PL 7 64 MiCOM P741 P742 P743 PL ...
Page 299: ...Programmable Logic P74x EN PL Na7 MiCOM P741 P742 P743 PL 7 65 PL ...
Page 300: ...P74x EN PL Na7 Programmable Logic PL 7 66 MiCOM P741 P742 P743 PL ...
Page 302: ...P74x EN MR Na7 Measurements and Recording MiCOM P741 P742 P743 ...
Page 324: ...P74x EN FD Na7 Firmware Design MiCOM P741 P742 P743 ...
Page 344: ...P74x EN FD Na7 Firmware Design FD 9 20 MiCOM P741 P742 P743 FD FIGURE 10 P74x SYSTEM OVERVIEW ...
Page 351: ...Firmware Design P74x EN FD Na7 MiCOM P741 P742 P743 FD 9 27 FD ...
Page 353: ...P74x EN CM Na7 Commissioning MiCOM P741 P742 P743 ...
Page 429: ...P74x EN MT Na7 Maintenance MiCOM P741 P742 P743 ...
Page 431: ...P74x EN MT Ma7 Maintenance MT 11 2 MiCOM P741 P742 P743 MT ...
Page 451: ...P74x EN TS Na7 Troubleshooting MiCOM P741 P742 P743 ...
Page 453: ...P74x EN TS Na7 Troubleshooting TS 12 2 MiCOM P741 P742 P743 TS ...
Page 475: ...P74x EN SC Na7 SCADA Communications MiCOM P741 P742 P743 ...
Page 499: ...P74x EN SC Na7 SCADA Communications SC 13 24 MiCOM P741 P742 P743 SC ...
Page 501: ...P74x EN SG Na7 Symbols and Glossary MiCOM P741 P742 P743 ...
Page 511: ...P74x EN SG Na7 Symbols and Glossary SG 14 10 MiCOM P741 P742 P743 SG Logic Gates ...
Page 513: ...P74x EN IN Na7 Installation MiCOM P741 P742 P743 ...
Page 515: ...P74x EN IN Na7 Installation IN 15 2 MiCOM P741 P742 P743 IN ...
Page 528: ...Installation P74x EN IN Na7 MiCOM P741 P742 P743 IN 15 15 IN FIGURE 6 P742 40TE REAR VIEW ...
Page 533: ...P74x EN VH Na7 Firmware and Service Manual Version History MiCOM P741 P742 P743 ...
Page 542: ...APPENDIX A WIRING DIAGRAMS ...
Page 543: ...Appendix A Wiring Diagrams P74x P74x EN M Na7 ...
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