Outputs and Commands - BCPro - LIT-12011908 - General System Information - BCPro System - 4.0

MS/TP Communications Bus for the BCPro System Technical Bulletin

Brand
BCPro
Product name
BCPro System
Document type
Technical Bulletin
Document number
LIT-12011908
Version
4.0
Revision date
2020-03-16
Language
English

Excessive SA Bus traffic can also result when a fast process sends commands to multiple AOs on the SA Bus. AO commands are sent every time the proportional-integral-derivative (PID) loop execution results in a new value. The CH-PCG can issue only 10 commands on the SA Bus per pass of the token. If excessive AO commands are generated, the output queue may fill up, and the issuance of commands may be delayed for several token loop times. In addition, the BACnet protocol engine in the controller may become overwhelmed and discard messages when the total pending queue count exceeds 50 messages. This state can result in AOs or the CH-PCX appearing offline for short periods of time.

A healthy SA bus should not exceed 20 for the Maximum Output Queue Used value because that value requires 2 token passes to issue the commands. Use the following table to determine the command rate per minute on the SA Bus. The goal is to have the total number of commands under the 500 per minute with a 1 second or less token loop time.

Table 1. Command Rates: Outputs

Process ID

Time Constant (Seconds)

PID Interval (Seconds)

Approximate Command Rate Per Minute

CHWS-T HEX

90

10

6

HWS-T HW HEX

90

10

6

HWS-T Steam HEX

90

10

6

DA-H

200

12

5

DA-T

200

12

5

MA-T

80

12

5

CWS-T

600

60

1

RA-H

900

60

1

ZN-H

900

60

1

ZN-T

900

60

1

ZN-T LC

900

60

1

ZN-H LC

900

60

1

For example, assume you have a Mixed Air Single Path, Variable Speed Supply fan with Volume Matching system. The control loops include discharge control, duct status pressure control, and return fan flow control. Use the following table to calculate the command rate per minute.

Table 2. Command Rate Calculation: Outputs

Control Loop

Process ID

PID Interval (Seconds)

Command Rate Per Minute

Duct Static Pressure Control

Duct Static

1

60

Return Fan Flow Control

Airflow

1

60

Discharge Control

DA-T

12

5

   

Total

125

When the outputs are located on SA Bus devices, 125 is the expected command rate per minute. This number is below the maximum of 500 commands per minute, and no communication issues result.