Specifications

Communications 3-5
if four modules are used in a chain operating at 1200 baud, the accumulated
delay time is 4 X 8.33 mS = 33.3 mS This time must be added to the times
listed in Table 3.1 to calculate the correct communications time-out error.
For modules with RS-232C outputs, the programmed communications
delay specified in the setup data (see Chapter 5) is implemented by sending
a NULL character (00) followed by an idle line condition for one character
time. This results in a delay of two character periods. For longer delay times
specified in the setup data, this sequence is repeated. Programmed
communications delay is seldom necessary in an RS-232C daisy chain
since each module in the chain adds one character of communications
delay.
Changing Baud Rate
It is possible to change the baud rate of an RS-232C daisy chain on-line. This
process must be done carefully to avoid breaking the communications link.
1. Use the SetUp (SU) command to change the baud rate setup on each
module in the chain. Be careful not to generate a reset during this process.
A reset can be caused by the Remote Reset (RR) command or power
interruptions.
2. Verify that all the modules in the chain contain the new baud rate setup
using the Read Setup (RS) command. Every module in the chain must be
setup for the same baud rate.
3. Remove power from all the modules for at least 10 seconds. Restore
power to the modules. This generates a power-up reset in each module and
loads in the new baud rate.
4. Change the host baud rate to the new value and check communica-
tions.
5. Be sure to compensate for a different communications delay as a
result of the new baud rate.
Using A Daisy-Chain With A Dumb Terminal
A dumb terminal can be used to communicate to a daisy-chained system.
The terminal is connected in the same manner as a computer used as a host.
Any commands typed into the dumb terminal will be echoed by the daisy
chain. To avoid double characters when typing commands, set the terminal
to full duplex mode or turn off the local echo. The daisy chain will provide the
input command echo.