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AS-Interface | The System | Experts Forum | FAQ
Overview
•AS-Interface System
◦What does AS-i mean and what can AS-i be used for?
◦What do AS-i Master and AS-i Slave mean?
◦What does "extended address mode" mean?
◦What are the ID-codes and what are the properties of the ID-code 1 and 2?
◦The AS-i IC in master mode checks the syntax of the incoming data. This data will be forwarded to the output. What happens in cause of an error?
◦How is the data transferred without any clock?
◦How does the Manchester-II-encoding work?
◦How long can the delay between data transfers be?
◦Which are the most significant parameters in an AS-i Network?
◦How is a data transfer of more than 4 bits organized, e.g. with intelligent sensors?
◦What are the characteristics of the first negative amplitude of a data package?
◦Slaves close to the master generate smaller amplitudes at the master than slaves located far away from the master. What is the cause?
◦What does it mean, that the slave outputs are "1" after start-up of the system?
•AS-Interface Applications
◦What is the maximum length of an AS-i network?
◦Can I install several repeaters in different network-arms?
◦How many slaves can I connect to the network?
◦An installed network does not work with a specific slave. What is the cause?
◦What is important when addressing a slave?
◦I want to change a slave address from 25 to 14. It does not work. What can be the cause?
◦A slave is correctly connected to a network, but I cannot exchange data with this slave?
◦What happens in case of a shorted AS-i bus?
◦How is the master powered in case of the shorted AS-i Network?
◦Is it possible to connect AS-Interface with GND (shield)?
◦What is the difference between master- and repeater mode?
AS-Interface System
What does AS-i mean and what can AS-i be used for?
AS-i (abbreviation for Actuator-Sensor Interface) is a bus system for low level field applications in industrial automation. Using the AS-i bus, a cost-efficient and quick connection of actuators and sensors is provided.
What do AS-i Master and AS-i Slave mean?
An AS-i Master communicates with AS-i Slave modules. The master exchanges information with slave modules, which are connected to the AS-i network. The master controls all activities on the bus. A slave will only respond, whenever a master request is directed to the slave. A slave forwards information of the connected I/Os.
What does "extended address mode" mean?
The extended address mode is a feature of the AS-i 2.1 specification. In that case it is possible to connect up to 62 slaves to the master. Using 62 slaves will increase the cycle-time up to 10ms. Note: The address space is still from 1 up to 31. One address may be assigned either to one slave in normal address mode or to one slave in extended address mode (A- or B-type) or to two slaves in extended address mode (A- and B-type).
What are the ID-codes and what are the properties of the ID-code 1 and 2?
The ID-codes describe the properties of a slave device. The ID-codes are specified in the AS-i specifications. By using the ID-codes it is possible to exchange different slave-modules from different manufactures while maintaining the "plug-and-play" features of AS-i. The extended ID-code 1 can be changed by the customer. A customer can program own parameters with ID-code 1. This ID-code includes information about the usability of this slave in the extended address-mode. The ID-code 2 is set by the manufacturer and gives information about the identity-properties of the slave.
The AS-i IC in master mode checks the syntax of the incoming data. This data will be forwarded to the output. What happens in cause of an error?
Every received data-telegram will be checked. In cause of a detected error the Manchester signal will be set to high. Additionally one pin of the parameter port will be set and no strobe signal will be supported. The connected control unit will recognise this error.
How is the data transferred without any clock?
The rules of the Manchester-II-encoding are the base of the data assignment. The clock results from the data-stream. Based on these transfer rules a synchronous data exchange will be supported.
How does the Manchester-II-encoding work?
The data will be transformed into edges. A rising edge in the middle of the bit time corresponds with a logical "1", a falling edge with a logical "0". After a positive data pulse always follows a negative data pulse. To encode the data stream, delay times (length of a half bit time) are inserted.
How long can the delay between data transfers be?
In order to access a slave there is a minimum period (cycle time) of 150µs required. In case of a network with 31 slaves the master will scan all slaves within a period of 5 ms and in case of 62 slaves (extended addressmode) the period will increase to 10 ms. Between transactions on the network cable there are pauses as following required:
•Master delay (delay time before a new master call): 2,5…5,5 bit times
•Slave delay (delay time between master call and slave respond): 1…2 bit times
Which are the most significant parameters in an AS-i Network?
Cable Power Supply Master Slave (std.) Slave (std.) Slave S-X.A Slave S-X.A (alt.)
R’ < 90mΩ/m > 5kΩ > 8kΩ > 8kΩ > 13.5kΩ > 13.5kΩ
Z 70…140Ω < 0.5Ω
G’ <= 5mS/m
C’ < 80pF/m < 400pF < 100pF < 70pF
+ (L - 6 mH)
* 10 pF/mH < 50pF < 35pF
+ (L’ - 12mH)
* 10pF/mH
L’ 400…13,000nH/m 100mH ±10% > 3mH > 9mH 6…9mH > 13.5mH 12…13.5mH
UASI 29.5…31.6V DC
How is a data transfer of more than 4 bits organized, e.g. with intelligent sensors?
The bit stream is divided in multiple sequences. Example: the first slave response gives the first nibble of a byte, followed by the next bits in the next cycle.
What are the characteristics of the first negative amplitude of a data package?
The threshold level for following pulses is set to 45-55% of the maximum of this first amplitude. Pulses with an amplitude of 80% (Slave) / 60% (Master) must be identified. Interferences with an amplitude of less than 30% have to be ignored.
Slaves close to the master generate smaller amplitudes at the master than slaves located far away from the master. What is the cause?
The modulated amplitudes are generated by the inductivity of the network. Because of different network inductivities on different places in the network, different amplitudes will be generated.
What does it mean, that the slave outputs are "1" after start-up of the system?
The in the output channel integrated open drain transistor is not switched on, that means the output is not connected to GND.
AS-Interface Applications
What is the maximum length of an AS-i network?
The maximum length of an AS-i network segment is 100m.
Repeater:
By using up to 2 repeaters, the network can be extended up to 300m. It is necessary to connect a power supply after each repeater.
Extender:
Using an extender is possible, if all the slaves are connected in the second network segment (e.g. at a distance of more than 100m from the master). It is forbidden to place slaves between the master and the extender.
Can I install several repeaters in different network-arms?
Yes, in that case it is possible to extend the network up to 500m (Master is located in the middle sector of the network).
How many slaves can I connect to the network?
In the basic address-mode it is possible to connect up to 31 slaves to one master. If the network master supports the extended address-mode, it is possible to connect up to 62 slaves which operate in extended address mode.
An installed network does not work with a specific slave. What is the cause?
The operation of 2 slaves with the same slave-address (1…31) gives a communication error. You have to change one of the slave addresses manually. Another malfunction will be provoked, when 2 slaves with the address "0" are connected. Slaves with address "0" must be installed one after the other (automatic addressing by the master is assumed). Slaves with the address "0" are not included in the data communication.
What is important when addressing a slave?
Attention should be paid to the address-lists of the master. It is important, that a slave matches the corresponding profile in the master. The installation of slaves at the physical layer (order of addresses) is not important. That means, a slave with the address 25 can be a neighbour of a slave with the address 14, followed by a slave with the address 30.
I want to change a slave address from 25 to 14. It does not work. What can be the cause?
To change a slave address, it is important to set the address to "0" first. Afterwards you can set the new slave address.
A slave is correctly connected to a network, but I cannot exchange data with this slave?
Before you can start the data exchange with a slave, you have to send the parameter data to this slave (Using the command "write parameter").
What happens in case of a shorted AS-i bus?
The master will give a power fail flag to the higher-level unit. After the error, the network will be restarted by the master.
How is the master powered in case of the shorted AS-i Network?
The master IC is electrically isolated from the higher-level unit. Therefore a separate power supply is always necessary. This makes it possible to forward the power-fail error to the higher-level unit.
Is it possible to connect AS-Interface with GND (shield)?
Never. The AS-i network is a symmetric network. This network must be electrically isolated from other components. The network requires an own power supply. This power supply must be a special symmetric supply, fulfilling the requirements of the AS-i specification. This power supply has a shield contact. An asymmetry to GND with more than 2% and an asymmetry of the connected network of more than 10% is not permitted.
What is the difference between master- and repeater mode?
The difference is the provision of error signals at the data and parameter ports. The higher-level unit can analyse these signals.
