CHAPTER 2: Introduction
9
2.4 Controller-to-Terminal Distance Limitations
The IBM controller has a timeout value after trans-
mitting a message to a terminal. If the response
from the terminal is not received within this
timeout, the controller ignores it and sends the
message to the next terminal.
2.4.1 D
ELAY
C
ALCULATIONS FOR
C
ONFIGURATION
1
The round-trip propagation time is equal to twice
the time it takes the signal to travel from the
controller (Tc) through segments X1, X2, X3,
and X4, and through multiplexers T1, T2, and T3,
plus terminal response time (see Figure 2-5).
Maximum cable length is calculated as follows:
X1+X2+X3+X4 = [Controller Timeout-
(Tc+T1+T2+T3+Terminal Delay)] x Signal
Propagation ÷ 2.
When:
Controller Timeout = 3174 - 50 µsec
3274 - 56 µsec
Data Rate = 424 nsec/bit
Tc (Controller Delay Time =
3174 - 3.5µsec (8-bit start sequence)
3274 - 24µsec (24-bit + 32-bit data stream)
T1 (MX330A-C Delay time) =
8-bit inbound, 8-bit outbound (6.78 µsec)
T2 (MX330A-D Delay time) =
16-bit inbound, 8-bit outbound (10.1 µsec)
T3 (Multiplexor Delay time) =
16-bit inbound, 8-bit outbound (10.1 µsec)
Terminal Delay (IBM max.) = 5.5 µsec.
Signal Propagation = 200 m/µsec
Applications using cable exceeding the maximum
calculated length require changing the controller
timeout. Timeout can be increased to 98 µsec or
200 µsec using an RPQ diskette, which can be
ordered from IBM.
MX330A-C
3174
Controller
or
3274
(with RPQ 9901)
MX330A-D
Copper
F/O
0 - 7
8 - 15
16 - 23
24 - 31
0 - 7
8 - 15
16 - 23
24 - 31
0
1
7
0
1
7
8-Port
Multiplexer
8-Port
Multiplexer
T
X
T
X
T
X
T
c
1
1
2
2
4
3
X
3
Figure 2-5. Delay Calculations for Configuration 1.
