This page documents my electrical and software interface between a Heatmiser PRN-EN V2 Thermostat and some code written in C#.

I've prepared this page to describe my setup in detail (to support my posting on : Comfort Forums) in the hope that someone with experience of the Heatmiser interface can assist me in understanding why the thermostat is not responding to commands on the RS485 interface. Update : 16 March 2012 : I recently got the thermostat to respond. There seems to be some buffering going on, either the UDS100 or the TCP stack, as bytes received by the UDS100 do not immediately appear on the the C# stream. I will investigate further as time allows to see what the cause is and update this page at that time.

First thing to check is that I really have a Version 2 Thermostat, as compared with a Version 3. Heatmiser have a page that describes the difference between the two. Photographs of the thermostat are available here : front and back.
Power supply : I use a simple mains transformer type power supply providing +12V DC, connected to the + and - screw terminals of the PRT-EN.
Data connections : This document (along with other sources online) states that Yellow should be connected to RS485 A(+) and Blue to B(-).
PC to RS485 Interface : I have a UDS100 acting as as bridge between RS485 and Ethernet. UDS100 Manual
RS485 Interface of UDS100 is configured for 4800bps, 1 stop bit, no parity,2 wire ; screenshots of the UDS100 configuration screens are here, here and here.
UDS100 appears on IP : 192.168.1.201 and port number 10002
In order to query the setpoint of thermostat with address 1, one sends the following four bytes to the UDS100 : 01H, 04H, 00H, 05H
I have checked that the thermostat is configured as having address 1.
When I do this with an oscilloscope on the Yellow or Blue wire I can see the bytes being transmitted.
I expect to see the bytes written (which I do), and then a short time later (<1000ms?) a response from the stat. However, I do not see the response on the oscilloscope.
The code I'm using to open a TCP connection to the UDS100, to transmit 4 bytes, to determine if bytes are received in reply, and to display any received bytes appears below at the bottom of this document.
Lantronix have a document which describes the pinout of the 25 pin connector and which connections to make for a 2 wire interface.
Confusingly, the above mentioned document states : Jumper pins 14 & 21. These pins become the positive pins and are connected to the positive (B) pin on your serial device." whereas I understood that conventionally positive is written as A.
I connect pin 7 (ground) of the Lantronix port to the - (power screw terminal) of the thermostat. Pin 14 (TX+ Out) to Pin 21 (RX+ In) to Heatmiser Yellow flylead. Pin 15 (TX- Out) to Pin 22 (RX- In) to Heatmiser Blue flylead.
As a check that my software is functional, if I put the UDS100 into 4 wire mode, and write bytes to the UDS100, immediately the software reports that bytes are waiting on the stream to be read, and reading 4 bytes from the stream yields the expected 4 bytes that have just been written to the UDS100. This is exactly what I would expect as the Transmit and Receive pins are hard wired together on my 25 pin male connector.
Termination resistors : I made the assumption that both the PRT-EN and UDS100 have approoriate resistors to present the correct impedance. Maybe this assumption is incorrect. I place a 120 Ohm resistor near the stat between the Yellow and Blue cables, but I still receive no reply from the stat.
Any advice / suggestions gratefully received.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Net;
using System.Net.Security;
using System.Net.Sockets;
using System.Net.Mail;
using System.IO;
using System.Threading;

namespace Heatmiser
{
    public partial class TestTCP : Form
    {
        TcpClient Client;
        NetworkStream stream;
        StreamReader reader;

        public TestTCP()
        {
            InitializeComponent();
        }

        private void TestTCP_Load(object sender, EventArgs e)
        {
            Client = new TcpClient();
            Client.Connect("192.168.1.201", 10002);
            stream = Client.GetStream();
        }

        private void buttonWrite_Click(object sender, EventArgs e)
        {
            byte[] wbuffer = new byte[] { 0x01, 0x04, 0x00, 0x05 };
            stream.Write(wbuffer, 0, wbuffer.Length);
            stream.Flush();
        }

        private void buttonPeek_Click(object sender, EventArgs e)
        {
            textBoxPeek.Text = stream.DataAvailable.ToString();
        }

        private void buttonRead_Click(object sender, EventArgs e)
        {
            char[] characters = new char[4];

            reader = new StreamReader(stream); 
            reader.Read(characters, 0, 4);
            StringBuilder sb = new StringBuilder();

            for (int i = 0; i < characters.Length; i++)
            {
                sb.Append(((byte)characters[i]).ToString());
                sb.Append("\r\n");
            }

            textBoxReadCharacters.Text = sb.ToString();
        }

        private void buttonWriteMultiple_Click(object sender, EventArgs e)
        {
            for ( int i = 0 ; i < 10 ; i++)
            {
                byte[] wbuffer = new byte[] { 0x01, 0x04, 0x00, 0x05 };
                stream.Write(wbuffer, 0, wbuffer.Length);
                stream.Flush();
                Thread.Sleep(20);
            }
        }
    }
}

On a totally unrelated topic, I replaced the BA92-07336A (BA92-07336B) (touchpad button PCB) in my Samsung RV711.

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