8-4 Field Control™ Genius® Bus Interface Unit User’s Manual – October 1999 GFK-0825F
8
Write Map
Subfunction Code: 2C hex
This datagram allows a CPU to change a previously-configured or unconfigured BIU network map
addresses, if configu ratio n is not currently protected. The BIU’s network map d efines the BIU memory
locations of th e data that is exchan ged on the bus. Star ting references in I, AI, Q, and AQ memory ar e
sent. If a ny length is zero, the associated starting reference is ignored; it is not meaning ful.
Byte No. Byte Description
0 Not used
1 Starting reference for discrete input ( I ) data (LSB)
2 Starting reference for discrete input ( I ) data (MSB)
3 Length of discrete in pu t ( I ) data (in byt es)
4, 5 Starting reference of analog input ( AI ) data
6 Length of analog input ( AI ) data (in bytes)
7, 8 Starting reference of discrete output ( Q ) data
9 Length of discrete output ( Q ) data (in bytes)
10,11 Starting reference of analog output ( AQ ) data
12 Length of analog output ( AQ ) data (in bytes)
13-15 Not used, but these bytes must be present for correct length message.

How a Write Map Datagram Affects I/O Modules in the Station

Receiving a Wr ite Map datagr am with differ ent startin g a ddresses and/ or lengths ch a n ges the
memory assigned previously configured for the BIU's network map, as explained above. Because
only data included with in the map is exchanged on th e bus, the BIU responds to such ch an g es in its
network map by automatically reassigning the I/O addresses of the modules in the station. This
assures that the I/O data wi ll continue to be exchanged on th e bus. It is important that th e len g th s
supplied in the Write Map datagram accommodate all of the data that should be exchanged. The
data of any modules configured outside the BIU's network map will be scanned by the BIU, but not
exchanged on the bus.
For example, suppose an I/O station has three 16-point discrete input modules, originally
configured to use addresses I0001 through I0048. All of this input data is included in the BIU's
network map and is sent on the bus.
Original Configuration: Starting reference: I00001, Length of I data: 6 bytes
I00001 I00009 I00017 I00025 I00033 I00041 I00048
slot 1 slot 2 slot 3
Later, a Write Map datagram is sent to the BIU, specifying a beginning address of I0065 and a
length of 5 bytes for I data. The BIU would automatically reassign starting addresses to the I/O
modules starting at I00065. However, this would place 8 inputs (I00105 through I00112) outside
the network map and the BIU would n o longer send those input s to th e host.
Write Map Datagram: Starting reference: I00065, Length of I data: 5 bytes
I00065 I00073 I00081 I00089 I00097 I00105 I00112
slot 1 slot 2 slot 3