General Service Guide
11
Document No. 896 623
2.4
Module communication
The communication master controlling the data transfer between CPU bus and module bus is
called UPI (Universal Peripheral Interface) board. It sends information or questions to each module
100 times per second, and if the module is present it replies to each question immediately by
sending a data package back to UPI board. This communication protocol ensures that each
module receives and sends information every 10 ms, the package length depends on the module
type. If the module does not respond, the UPI presumes that the module is not connected.
Each module type has a unique name, which UPI uses to address its messages. If there are two
modules of the same type, they both answer at the same time. This results an error and thus two
similar modules are not allowed in one frame.
Module Bus
Microcontroller
A
S/3 Bu
s
Dual-port
RAM
UPI Board
CPU Bus
Figure 4
Principle of UPI operation
The microprocessor on the UPI board collects all information coming from the modules into a dual-
port RAM. This memory is located on UPI board and it is mapped directly to the address space of
the main CPU. The main CPU is thus reading information from it's own memory and UPI guarantees
that the data is up to date. This operation works also to the other direction, when CPU is filling the
dual port RAM with data and UPIs’ microprocessor is distributing it to the modules.
