TTDM–NMM and - California Detection Systems

Appendix 7 – Modbus Implementation
Overview
The TraceTek TTDM-NMM module can communicate with host systems through its serial port. The TTDM-NMM follows the
Modbus communication protocol to allow easy integration into building or facility management systems. The module acts as a
slave to the host system, and provides remote access to all data and settings, including the following:
• Event status and history
• System status
• Status of individual SIM channels
• Setup functions
Modbus Communications
As noted in Appendix 6, the TTDM-NMM external communications serial port can be configured for use either as an
RS-232 or RS-485 transceiver. The table below summarizes the standard configuration and options.
Standard configuration
Options
RS-232 RS-485 (selectable with jumpers; see p. 35)
Full-duplex, no hardware handshaking
9600 baud 600 to 19200 baud (selected from TTDM Network menu)
8 data bits, no parity, 1 stop bit
The TTDM-NMM supports both Modbus transmission modes: ASCII (American Standard Code for Information Interchange) and
RTU (Remote Terminal Unit). The TTDM-NMM must be set to the transmission mode used by the Modbus master (the host system);
to select the TTDM-NMM transmission mode, use the Modbus selection in the TTDM Network menu. Both modes communicate
the same information, but format messages differently, as summarized in the table below.
Transmission Beginning of
mode message frame Data format Error check End of Frame
ASCII : Hexadecimal, ASCII LRC* Carriage return and line feed
characters (16 bits)
RTU Silence (3.5 8 bit binary CRC** Silence (3.5 character times)
character times)
* LRC = Longitudinal Redundancy Check
**CRC = Cyclical Redundancy Check
Supported Modbus Functions
The TTDM-NMM provides the Modbus master (host system) with access to all functions that would be available using the keypad
on the TTDM-NMM unit itself. The table below shows how this access is implemented using Modbus functions (identified by
Modbus function code and location reference). For an explanation of the Modbus protocol, consult the web site of Modicon (a division
of AEG Schneider Automation, Inc.) at http://www.modicon.com, and/or their publication “Modicon Modbus Protocol Reference
Guide (PI-MBUS-300).”
Modbus functions
Type of data
Write Write Location Modbus Field
Read (single) block reference description TTDM-NMM usage size*
1 5 15 0XXXX Coils Global commands and module settings 2
2 ‡ ‡ 1XXXX Digital inputs Alarm relay/LED status 2
4 ‡ ‡ 3XXXX Input registers Event and status data 4
3 6 16 4XXXX Holding registers Query selections (SIM channel, event number) 4
Module settings
* Number of digits, ASCII encoded headecimal; half that number of digits in RTU (binary) mode.
‡ Digital inputs and input registers are read-only; there are no Modbus write functions for those registers.
Important: Maximum block size for a query/response is 16 coils, inputs, or registers in the TTDM-NMM.
35