SA Bus rules and specifications - Metasys - LIT-12011034 - General System Information - Metasys System - Release 11.0

MS/TP Communications Bus Technical Bulletin

Product name
Metasys System
Document type
Technical Bulletin
Document number
Release 11.0
Revision date

The SA bus connects IOMs, XPMs , VSDs, and NS-Series network sensors to equipment controllers. Figure 1 in End-of-Line termination on SA Bus shows three SA bus examples. Table 1 provides SA Bus rules and limits. Table 2 provides SA Bus specifications.

Table 1. SA Bus Rules




Each bus supervisor supports one SA Bus (and each SA Bus is a single segment).

Number of devices supported on the SA Bus

The guideline for the maximum total number of devices supported on an SA Bus is 10. The SA bus supervisor provides power for all subordinate devices connected to the SA bus, including up to eight NS network sensors. IOMs and XPMs connected to the SA bus are not powered by the bus supervisor.

SA Buses do not support repeaters. See Table 1 for SA Bus power consumption.

Cable length

365 m (1,200 ft) maximum bus length

152 m (500 ft) maximum distance between an NS network sensor and the bus supervisor (CGM, CVM, FAC, FEC, or VMA supplying power to the sensor) using bus cable connected to the SA Bus screw terminal blocks

30 m (100 ft) maximum length for network sensors using bus cables connected to the RJ-Style modular jack (6-Pin SA Bus Port)

366 m (1,200 ft) maximum bus length

Recommended cable type

Screw Terminal Connections: 22 AWG Stranded 4-wire, 2-Twisted Pairs, Shielded Cable for screw terminals.

Modular Jack Connections: 6-Pin RJ-Style Modular Connectors with 24 or 26 AWG Solid 6-Wire, 3 Twisted-Pairs

EOL termination

Each SA bus supervisor has integral (fixed ON) EOL termination, which typically provides sufficient EOL termination on an SA bus. Long SA bus runs or persistent communication problems on an SA bus may require EOL termination at the last device on the SA bus (in addition to the integral EOL termination at the SA bus supervisor).

Mixing device types

Do not mix RJ-style modular (phone) jack devices and screw terminal devices on the SA bus.

Note: The recommended cable types provide the best bus performance. See MS/TP Bus cable recommendations for information on alternative cable types.
Table 2. SA Bus Specifications
Category Specification
Error checking Message Headers checked using 8-bit CRC test. Message data check using 16-bit CRC test.
Device addressing 0-255 (See Device addressing on the MS/TP Bus for more information.)
Data transmission standard RS485
Signaling method BACnet® MS/TP
Signaling rate 9600; 19,200; 38,400 (default); or 76,800 baud as selected by the bus supervisor
Transient immunity Meets EN61000-4-4 and EN6100-4-5 requirements for heavy industrial applications. Protected against misapplication of 24 VAC.
Shield grounding One hard ground per bus segment when using shielded cable
Physical configuration Daisy-chained (screw terminal only).