CAS Modbus Explorer
Modbus communications not working ? How do you know what’s wrong – is it the baud rate, the parity or even the address? There are so many things that can affect communications.
The Modbus protocol does not support discovery. There is no broadcast message that you can send that will make a slave respond with its port and address settings. The protocol allows for broadcast messages to be sent but doesn’t demand that every vendor supports this feature.
The tool works for RS232 and RS485 by allowing you to search for a single device or multiple slaves. To connect to a RS485 network from your laptop you simply need a USB-to-485 converter.
Here is the problem – there are thousands of combinations:
Baud 300, 600, 1200, 2400, 4800, 9600, 19200, 38400, 57600, 115200 Parity, None, Odd, Even Stop Bits 1, 2 Data Bits 7, 8 Address 1-254
If you had to manually search/test all these combinations it will take you ages. Relax, go for lunch and let the CAS Modbus Explorer do all the work for you.
IN THIS ISSUE
Probably not the best of ideas:
RS485 on a Scope
Using an oscilloscope on a RS485 network. For reference purposes we present some scope captures of a typical RS485 line.
Zone A: Idle State
Zone E: Idle State with Noise
Zone B: The device has enabled it transmitter but has not started transmitting. The line is driven to a known state. The duration of this phase can be controlled by configuration in some devices. If it is too short then it is possible that some of the front of the message may be lost.
Zone C: Unless you have a very high speed scope and the scope can takes a huge number of samples you are unlikely to see each bit in the message. In this capture we can see the bytes (roughly speaking) but not the bits. We were forced to accept this compromise because the number of samples we could capture at a sampling rate high enough to see the bits would mean that we could only capture the 1st couple of bytes of the message and we would have to set the trigger to ignore zone B. Except with a very expensive scope you will be unlikely to be able to see the whole message and all the bits.
Zone D: We have finished transmitting but the transmitter is still enabled. Normally the device should disable its transmitter as soon as possible after transmitting the last stop bit of the message but since that can be difficult to achieve in the hardware, many devices run a timer to make sure they don’t disable the transmitter too soon. The problem with this approach is:
Beer and Vodka Can Help You Select a Terminating Resistor
Try this mnemonic if you are trying to remember the resistor color codes:
Note: If you’re missing a tolerance band that implies that the tolerance is 20%.
Which end do you start reading the color bands?
There are usually two ways:
1) If one of the bands at the end of the sequence is further apart then that is the tolerance band – start from the opposite end.
What should you carry with you to site? (for communication networks purposes)
How to buy resistors
How to make a resistance value even if you don’t have the correct resistor in your toolbox.
By placing resistors in series or parallel you can create new values.
Resistor color codes for 4 and 5 band resistors.
Federal Signal Ultravoice
We have a new FieldServer driver for Federal Signal Ultravoice (FSU) – Electronic Siren Controllers (ESC).
The driver can monitor the status of the Ultravoice devices and send commands to perform functions such as ARM, WAIL, CODE, MSG and many more.
Free CAS BACnet Explorer
Add $795* value to your FieldServer for free. Simply purchase a FieldServer with a BACnet protocol from CAS and we will provide you with a free (and transferable) license to our famous BACnet Explorer. Use the explorer to test and commision the installation and then use it on your next job or pass it on to your customer.
This is a full license and includes the USB key which allows you to transfer your license from laptop to laptop with no hassles. You get all updates to the product for a period of 1 year.
2008 Chipkin Automation Systems – firstname.lastname@example.org
BACnet is a registered trademark of ASHRAE.