RS232 Connection Notes
See the RS232 reference to determine the wiring for either a 9-way or 25-way female D Type "sub-miniature" connector.
RTS and CTS are connected together. RXD is used to steal a negative supply.
You may have to connect DSR and DCD to DTR if you are using the Arbiter hardware with other software. This connection is not required with HotBox or the NT service tempscv.exe.
LED's - what they indicate
L1 steady - power on
L2 flashing once a second - firmware is running
L3 lit - A DS1820 is being polled
If RTS drops, the controller will stop polling and turn off L2 and L3.
Disconnect from PC and disconnect DS1820 probes. L1 should be on, L2 flashing.
Connect a DS1820 probe. L3 should flick on once every 5 seconds. Additional probes can be connected "hot". (Each single wire bus is scanned every 5 seconds, additional devices will be recognized when connected.)
Connect to PC. L2, L3 out. Use (hyper) terminal set to 19200, 8 data, no parity, 1 stop bit. When "connected" (RTS/CTS asserted) L2, L3 should revert to flashing. Check for data frames.
PICS are general purpose micro controllers, manufactured by Microchip Technology. They are popular devices for hobbyists/ nerds/ electronic geniuses and can be used for all sorts of purposes.
PIC's come in different packages, speeds, voltage range, temperature range and power dissipation. The 16F84-04/P is the "entry level" version. Its siblings will probably work in the circuit, but I haven't tested all of them. Avoid /SO devices - unless you've laid out your board for surface mount!
A 16F84-04/P device needs programming before it can do anything useful. The PIC Source .asm file needs to be compiled. The generated .hex file will contain the program instructions and configuration bits. The .hex file is read by the programmer and is used to "blow" the device. A 16F84-04/P can be reprogrammed many times.
How long a cable run can I have?
I'm using several cable runs around my house. One run is 30 metres, and has 8 DS1820's connected to it. There is a noticeable "droop" when looking with a 'scope - but the probes work fine. I'm using twisted pair phone cable.
See this Dallas application note: Guidelines for Reliable 1-Wire Networks
Is the crystal frequency critical? Can I use a 4MHz crystal?
No - the 2.4576 MHz frequency is required for accurate baud rate timing. The frequency is 27 * 19200 Hz.
Why not use a MAX232 RS232 chip?
It would work fine! I just had a PNP transistor to hand - so I used it. A Max232 would be better if you are planning to drive a long RS232 cable.
Your PIC source is in assembler. Why not C?
No real valid reason - just to be different! C# one minute, assembler the
next, that's me.
How does the USART serial timing work?
This is explained here.
The Arbiter can handle 64 temperature sensors. Where do I connect them?
Up to 8 DS1820 devices can be connected to each Port B pin. There are 8 Port B I/O pins, giving the total of 64.
I can't obtain the PIC16L84-04/P. Is a PIC16F84 OK ?
Yes. A PIC16F84-04/P has been tested and is a replacement.
The PIC16F84A-20/P has also been tested.
Avoid ordering the /SO devices, unless you are planning to use surface mount technology!
My PC doesn't have any serial ports? What can I do?
Get a USB to RS-232 adapter.
Don't skimp! there are some pretty inferior adapters out there with really dodgy drivers. You'll get frustrated and lots of blue screens of death.
Stump up £12.50 and order a USB to RS-232 adapter such as the "USB-COM-PL" from Easysync.
How do I compile the .ASM file so I can program the PIC?
What are the Farnell Part numbers?
The key parts are:
Printed Circuit Board Plots
Phil Tilson has sent revised PCB layouts. These look pretty tidy, and there's an alternative layout using a MAX232.
Karl Jan Skontorp has built up an arbiter using a printed circuit layout. He has kindly submitted his layouts.
This page was last updated on the 2nd May 2009.