Modbus Communication – FPS Nitrogen Generator – South-Tek Systems

Applies to: FPS-1650 · FPS-3250 · FPS-5000 · FPS-10000 · FPS-16500 · FPS-22500 (SMART-Trak™ touchscreen models equipped with the Ethernet / Modbus card)

This article explains how to integrate a South-Tek FPS fire protection nitrogen generator with a PLC, SCADA system, or BMS over Modbus TCP. It applies to the touchscreen FPS models that carry the Unitronics controller and Ethernet card. For hardwired alarm signaling instead of, or alongside, Modbus, see the dry-contact wiring article linked at the foot of this page.

ℹ Note The FPS register map is not the same as the industrial PSA generator map. Several points differ, including the Start / Stop coil. Use this article for FPS units and the industrial Modbus article for industrial PSA units. Do not cross-use the two.

1) Protocol and connection

Protocol: Modbus TCP/IP
Port: 502
Device role: Slave (the generator PLC is the slave; the JACE / SCADA / BMS is the master)
Physical layer: Ethernet, static IP required

The Communication Settings screens on the touchscreen are used to assign the static IP and confirm the link. The Network Communications Status screen reports Ethernet Card Exists: Yes, which is the fastest way to confirm a unit is equipped for Modbus before you start the integration.

2) What you need from the customer or network

Generator PLC static IP address
Routing so the master can reach the generator (same VLAN/subnet, or proper routing in place)
TCP port 502 allowed between the master and the generator PLC

3) Addressing and data format (read this first)

Addressing mode varies by client software. The table below lists holding registers as “4xxxx” and coils as “0xxxx”, and the addresses carry an offset of 1. Many drivers (including Niagara / JACE) want a zero-based offset instead. Confirm which convention your driver uses and adjust, or every point will read one register off.

Listed address	If your driver uses zero-based offsets	Example
Holding register 40006	Offset 5	40006 → 5
Holding register 40119	Offset 118	40119 → 118
Coil 00355	Offset 354	00355 → 354

Data types:

16-bit holding registers: start with Unsigned/Int16. If a value looks scaled (e.g. 2150 instead of 21.50), apply scaling in the client (for example, multiply by 0.01).
32-bit holding registers (run hours and leakage averages): use a 32-bit integer type (DINT/UDINT). If the number is wrong, swap word order (endianness) in your driver.

4) Register map

Holding Registers (16-bit)

Tag / Point	Unitronics	Modbus Address	Units	Access
Nitrogen Storage Pressure	MI 118	40119	PSIG	R
Oxygen Concentration (%)	MI 5	40006	% O2	R
Generator Cut-In Pressure	MI 20	40021	PSIG	R/W
Generator Cut-Out Pressure	MI 21	40022	PSIG	R/W
Bypass Cut-In Pressure	MI 254	40255	PSIG	R/W
Bypass Cut-Off Pressure	MI 259	40260	PSIG	R/W
Low Tank Pressure Alarm Point	MI 98	40099	PSIG	R/W
Low Tank Pressure Cut-Out Point	MI 346	40347	PSIG	R/W
O2% Alarm Point	MI 14	40015	% O2	R/W
O2% Alarm Cut-Out Point	MI 3	40004	% O2	R/W

Holding Registers (32-bit)

Tag / Point	Unitronics	Modbus Address	Units	Access
Generator Total Run Hours	ML 14	28687	Hours	R
Average N2 Leakage for Last Standby	ML 44	28717	SCFH	R
Average N2 Leakage for Today	ML 68	28741	SCFH	R
Average N2 Leakage for this Month	ML 70	28743	SCFH	R
Particulate Filter Run Hours	ML 46	28719	Hours	R
Coalescing Filter Run Hours	ML 47	28720	Hours	R
Absorbing Filter Run Hours	ML 48	28721	Hours	R

Coils (Discrete)

Tag / Point	Unitronics	Modbus Address	States	Access
Start / Stop	MB 354	00355	0 = Off, 1 = On	R/W
Running / Standby	MB 7	00008	0 = Standby, 1 = Running	R
Common Alarm	MB 61	00062	0 = Good, 1 = Alarm	R
Oxygen Concentration Alarm	MB 0	00001	0 = Good, 1 = Alarm	R
Low Tank Pressure Alarm	MB 106	00107	0 = Good, 1 = Alarm	R
BlastOff Alarm	MB 2050	02051	0 = Good, 1 = Alarm	R
Automatic Bypass†	MB 315	00097†	0 = Normal, 1 = Bypass	R
Particulate Filter Status	MB 14	00015	0 = Good, 1 = Change Needed	R
Coalescing Filter Status	MB 28	00029	0 = Good, 1 = Change Needed	R
Absorbing Filter Status	MB 29	00030	0 = Good, 1 = Change Needed	R
Low Battery Indicator	SB 8	20489	0 = Good, 1 = Low Batt	R
Local Alarm Buzzer Toggle	MB 227	00228	0 = Off, 1 = On	R/W
Low Tank Pressure Alarm Toggle	MB 386	00387	0 = Off, 1 = On	R/W
BlastOff Alarm Toggle	MB 2051	02052	0 = Off, 1 = On	R/W
Automatic Bypass Toggle	MB 2062	02063	0 = Off, 1 = On	R/W
Automatic Bypass Manual Activation	MB 382	00383	0 = Off, 1 = On	R/W

† Automatic Bypass status address pending engineering confirmation. The listed coil does not follow the +1 offset used by every other point in the map; confirm before relying on it.

ℹ Note — Filter Alarm The FPS reports the filter (“Change Filters”) condition per element, not as one combined bit. To surface a filter alarm to a BMS, map the three filter-status coils: Particulate (00015), Coalescing (00029), and Absorbing (00030), each 0 = Good, 1 = Change Needed. The touchscreen’s “Change Filters” status is just an OR of these three. For a single rollup point instead, the Common Alarm (00062) coil trips on any active alarm, and the filter run-hour registers can be trended toward the 1000-hour service interval.

▲ Attention This map is for FPS fire protection generators. It differs from the industrial PSA generator map. Most notably, Start / Stop is coil 00355 on FPS (it is 00013 on industrial PSA units). Confirm you are using the FPS map for FPS units before writing any command coil.

5) Example client setup

Channel: Modbus TCP/IP | Device: FPS_PLC (IP = generator static IP, Port = 502, Unit ID typically 1)

N2_Storage_Pressure
  Register Type: Holding Register
  Address: 40119   (or offset 118)
  Data Type: 16-bit Integer (Unsigned first); scale 0.01 if value reads 100x high

O2_Percent
  Register Type: Holding Register
  Address: 40006   (or offset 5)
  Data Type: 16-bit Integer

Filter_Status_Particulate
  Register Type: Coil
  Address: 00015   (or offset 14)
  Data Type: Boolean  (1 = Change Needed)

Start_Stop_Command
  Register Type: Coil
  Address: 00355   (or offset 354)   <-- FPS coil, not 00013
  Data Type: Boolean

6) Quick troubleshooting

Cannot ping the generator IP: check the Ethernet link, switch, and VLAN; confirm the static IP, subnet, and that the Ethernet card shows present on the Network Communications Status screen.
Ping works, no Modbus connection: confirm TCP port 502 is open end to end (firewall/ACL) and the master is pointed at the correct IP.
Connected, values look wrong: check the addressing mode (listed 4xxxx/0xxxx vs zero-based offset), 16-bit vs 32-bit data type, scaling on decimals, and word order on 32-bit values.
Cannot write Start/Stop: verify write permissions, the correct FPS coil address (00355), and that the unit is not held in a local or interlocked state.

Need help? South-Tek Systems Service — Phone: (910) 415-1880 · Open a support ticket at support@southteksystems911.zendesk.com or through the Help Center.