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Chapter 5
Application Notes

V Maximum Query / Response Parameters

V Estimating Serial Transaction Timing

V Application Notes for the 584 and 984A / B / X

5.1 Maximum Query / Response Parameters

The listings show the maximum amount of data that each controller can request or send in a master query, or return in a slave response. All function codes and quantities are in decimal.



These values are for an 8K controller. See the 484 User's Guide for limits of smaller controllers.





5.2 Estimating Serial Transaction Timing

The following sequence of events occurs during a Modbus serial transaction. Letters in parentheses ( ) refer to the timing notes at the end of the listing.

1 The Modbus master composes the message.

2 The master device modem RTS and CTS status are checked. (A)

3 The query message is transmitted to the slave. (B)

4 The slave processes the query message. (C, D)

5 The slave calculates an error check field. (E)

6 The slave device modem RTS and CTS status are checked. (A)

7 The response message is transmitted to the master. (B)

8 The master application acts upon the response and its data.

Timing Notes

A If the RTS and CTS pins are jumpered together, this time is negligible. For J478 modems, the time is about 5 ms.

B Use the following formula to estimate the transmission time:

Time (ms) = 1000 * (character count) * (bits/character)
Baud Rate

C The Modbus message is processed at the end of the controller scan. The worst-case delay is one scan time, which occurs if the controller has just begun a new scan. The average delay is half the scan time.

The time allotted for servicing Modbus ports at the end of the controller scan (before beginning a new scan) depends upon the controller model. Timing for each model is described on the next page.

For 484 controllers the time is approximately 1.5 ms. The Modbus port is available on a contention basis with any J470 / J474 / J475 that is present.

For 584 and 984 controllers the time is approximately 1.5 ms for each Modbus port. The ports are serviced sequentially, starting with port 1.

For 184 / 384 controllers the time varies according to the amount of data being handled. It ranges from a minimum of 0.5 ms to a maximum of about 6.0 ms (for 100 registers), or 7.0 ms (for 800 coils). If a programming panel is currently being used with the controller, the Modbus port is locked out.

D Modbus functions 1 through 4, 15, and 16 permit the master to request more data than can be processed during the time alloted for servicing the slave's Modbus port. If the slave cannot process all of the data, it will buffer the data and process it at the end of subsequent scans.

The amount of data that can be processed during one service period at the Modbus port is as follows:

Note: 984-X8X refers to 984 slot mount models (984-385, -685, etc).

For the 884, the processing time for multiple data is as follows:

E LRC calculation time is less than 1 ms. CRC calculation time is about 0.3 ms for each eight bits of data to be returned in the response.

5.3 Notes for the 584 and 984A / B / X

Baud Rates

When using both Modbus ports 1 and 2, the maximum allowable combined baud rate is 19,200 baud.

Port Lockups

When using ASCII, avoid sending zero-data-length messages or messages with no device address. For example, this is an illegal message:


:  CR  LF			(colon, CR, LF)
Random port lockups can occur this kind of message is used.

Terminating ASCII Messages

ASCII messages should normally terminate with a CRLF pair. With the 584 and 984A/B/X controllers, an ASCII message can terminate after the LRC field (without the CRLF characters being sent), if an interval of at least 1 s is allowed to occur after the LRC field. If this happens, the controller will assume that the message has terminated normally.





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        2000, 2006            Última modificación: 02/03/2006