Campbell MD9, Instruction Manual

Page: 10 / 22

MD9 Multidrop Interface
6
COM200
FIGURE 6-1. Telephone to MD9 Conversion
7. Operation
Communication between the computer and a datalogger is initiated and
controlled by the computer. The MD9 at the computer is activated when Pin
20, Data Terminal Ready (DTR), is pulled high on the computer's I/O port.
The signal is passed through the SC532(A) and in turn brings Pin 5 Modem
Enable (ME) high on the MD9s I/O port.
When the ME line is pulled high, the MD9 goes from State 0 standby to State 1
active. Once in State 1, any information transmitted by the computer is passed
on to the coax network. This remains true until the DTR and ME line are pulled
low and the MD9 reverts to State 0.
Immediately after the computer enables its MD9 it must send a "BREAK"
command followed by the "ADDRESS" of the destination MD9. See the
timing diagram in Appendix C.
Upon detecting activity on the coax line, remote MD9s convert from State 0 to
State 2. State 2 is a sleep mode in which the MD9 begins a timing routine to
determine if a BREAK has occurred. If a BREAK is not detected, the MD9
returns to State 0. If a BREAK is detected, all remote Multidrop Interfaces
capture and read the ADDRESS.
The properly addressed MD9 progresses to State 3 in which it pulls the
datalogger ring line and buffers any bytes of information which followed the
ADDRESS. All other remote MD9s alternate between States 0 and 2 trying to
identify whether the continuing activity on the network is a new BREAK.
When the datalogger detects the ring line signal it enters its telecommunication
mode and pulls its ME line high causing the MD9 to enter State 1. The link is
The null modem on the PS512M has
two 9-pin connectors. The SC12
cable from the COM200 is connected
to either connector and the SC12 from
the MD9 is connected to the other.