TB Controls TC02 Operation & maintenance manual - page 19
Operations & Maintenance Manual
January 2002
Doc: TC02 O&M
Page 19 of 20
Status:
Final
issue: 11
©TB Controls 2011
8.
MODBUS protocol and unit data
The hardware interface is half duplex RS485 (single twisted pair) and we normally use a ¼ unit load driver chip
with +/-60 volt line protection (LT1785CN8).
There are four screw terminal connections on the board these are RS485 B, RS485 A, Shield In, and Shield
Out. Either or both Shield In and Shield Out can be connected to local ground via a jumper link. Normally only
the Shield Out is linked to local ground.
The open protocol is MODBUS RTU at 19200 baud (default), no parity, 8 data bits, and 1 stop bit. Each unit has
a network address ID from 1 to 247 (8bit) settable from the LCD interface.
The following MODBUS function codes are implemented…
Read Holding Registers (3)
Write Single Register (6)
The following data is available as 16bit signed integers…
Tag no.
0. Off coil temperature: in ¼ ºC
1. Return air temperature: in ¼ ºC (if fitted)
2. Outside air temperature: in ¼ ºC (if fitted)
3. Electric heater element duty: in % or hot water valve position in % open (0-100)
4. Fan speed: 0-4 (0=Off, 4=Maximum) (R/W)
5. Summer/Winter mode: 0=Summer, 1=Winter (R/W), 2=automatic (Read only)
6. Summer Off Coil Set-point: range 10 to 40ºC. Default 18ºC (R/W)
7. Winter Off Coil Set-point: range 40 to 70ºC. Default 40ºC (R/W)
8. Summer Return Air Set-point/Trip: range 10 to 40ºC. Default 16ºC (R/W)
9. Winter Return Air Set-point/Trip: range 10 to 40ºC. Default 25ºC (R/W)
10. Summer Outside Air Set-point: range 5 to 40ºC. Default 20ºC (R/W)
11. Winter Outside Air Set-point: range 5 to 40ºC. Default 15ºC (R/W)
12. Stat Offset or Return Air setback: 0ºC to 30ºC. 0ºC is off (R/W)
13. Occupancy Timer: time to keep the unit running after a occupancy signal is detected, 5 to 60 minutes
in 1-minute steps (R/W)
14. Fan Run On time: 0 to 15 minutes in 1-minute steps (Default 3 minutes) (R/W)
15-23 Error/Status bits readable from the master unit (15 master and 16-23 slave units 0-7)
Bit 0: BMS/Time Switch input active, Unit OFF (Remote disable)
Bit 1: Over Temperature Trip
Bit 2: Off Coil Sensor Fault
Bit 3: Unoccupied Unit OFF (PIR sensor inactive for Occupancy timer period)
Bit 4: Return Air Sensor Fault
Bit 5: Outside Air Sensor Fault (master only, tag 15 only)
Bit 6: Return air sensor setback active or Stat input trip
Bit 7: spare
Bit 8: Slave comms fault (16-23 only, occurs only when a slave has been connected)
Bit 9: Slave connected (16-23 only)
A master unit uses bits 8 and 9 in tag registers 16-23 to indicate if a salve is connected or has become
disconnected. Slave units can also be addresses on the MODBUS network with tag registers 0-15 available as
read only data and tag 15 having bits 0-4 and 6 available.
Tag registers marked (R/W) are Read and Write.
A temperature value of 396, equivalent to 99ºC indicates a sensor not fitted or sensor fault, the appropriate bit
will also bet set in the status register (tags 15-23) if this is a fault condition.