3
RG-MHBAC
Introduction
2
Introduction
This document will explain how to operate an MCH Drive using BACnet protocol on a Master-Slave/Token-
Passing (MS/TP) LAN. It is intended as a serial communications supplement only and will not discuss
normal drive operations. For more information regarding normal drive setup and functionality, refer to the
MCH Installation and Operation Manual (MH01). Some of the information contained in this document was
provided by the ANSI/ASHRAE 135-2001 standard. Refer to http://www.bacnet.org for more information.
2.1 MS/TP Communications
MS/TP communication is maintained by a Master & Slave relationship between the devices on the LAN.
All communication is initiated by Master devices. The token passes network mastership from one Master
device to another. Slave devices never initiate a service request. They only respond to service requests
from master devices. The Native BACnet implementation in MCH Drives is for an MS/TP Slave device.
Therefore, only devices that support static device binding will be able to communicate with an MCH drive.
For an MS/TP EIA-485 network use shielded, twisted-pair cable with a characteristic impedance between
100 and 130 ohms. Maximum recommended length of an MS/TP segment is 1200 meters (4000 feet)
with AWG 18 cable. Maximum number of nodes (unit loads) per segment is 32 (as specified by the EIA-
485 standard). Because MS/TP uses non-return to zero (NRZ) encoding, the polarity of the connection to
the cable is important. The non-inverting input of the EIA-485 transceiver is designated as “+” and the
inverting input as “-”. Connect a termination resistance of 120 ohms (± 5%) at each of the two ends of the
segment medium. Ground the shield at one end only to prevent ground currents from being created. Each
MCH drive is equipped with 100k ohm local bias resistors.
2.2 MS/TP Physical Layer
Communication is half duplex. Bytes are transmitted using NRZ encoding with 1 start bit, 8 data bits, no parity
and 1 stop bit. All bytes are transmitted with the least significant bit first. Communication is asynchronous
with no more than 20 bit times of idle time between any two bytes of a frame. The communication baud
rate for MCH Series Drives is 9600 bps. The bit sequence is as follows:
DATA
START
1
2
3
4
5
6
7
8
STOP
2.3 Serial Communications Wiring
Figure 1 illustrates the MCH Series Terminal Strip and connections for an MS/TP LAN.
2
1
5A
5B
6
10A 10B
2
13A
12A
13B 13C 13D 14
15
2
TXB
RXA
16
17
18
EIA-485 -
EIA-485 +
Earth
Ground
Figure 1: MCH Terminal Strip
Connect
EIA-485+
terminal to MCH terminal TXB
Connect
EIA-485-
terminal to MCH terminal RXA
Connect MCH terminal 2 to Earth Ground.
