Rack Monitor 8800E/8801E
Document: Operation Manual
Date: 16-Jun-2011
Version: A.6
Page 25 of 35 H71343
Communication settings.
Before data communication can be used four switches must be set (two sets).
The two left switches are for terminating the communication lines with 120
Ω
On (up) = terminated with 120
Ω
Off (down) = NOT terminated
Both outgoing YZ as well as incoming AB channels are connected together when the two right switches are in the
On (up ) position.
Switch colour
Default
Function
Remarks
•
1 Brown.
Off
Terminator Y-Z
Off = none
On =120 Ohm *
3
•
2 Red
Off
Terminator A-B
Off = none
On =120 Ohm *
3
•
3 Orange
Off
Full/Half Duplex
Off = Y-A open
On = Y-A Short *
4
•
4 yellow
Off
Full/Half Duplex
Off = Z-B open
On = Z-B Short *
4
*
3
Switch 1 & 2 must be both on or both off.
*
4
Switch 3 or 4 must set to terminate both lines with one 120
Ω
.
Full Duplex:
The 8800E/8801E use terminal Y & Z for transmitting data and A & B for receiving data.
Half Duplex:
Since Y & Z are connected with A & B the 8800E/8801E will use A(Y) & B(Z) for transmitting
AND
receiving.
RS-485 (EIA-485)
RS-485 Balanced (differential) interface, Defines the Physical layer,
signaling protocol is not defined
. RS-485
specifies bidirectional, half-duplex data transmission. RS-485 can be made full duplex (like RS-422) however, since
RS-485 is a multi-point specification, it is not necessary in many cases.
Up to 32 transmitters and 32 receivers may be interconnected in any combination, including one driver and multiple
receivers (multi-drop), or one receiver and multiple drivers. The maximum of 32 devices is defined based on the
Unit Load [UL] of each device [12K ohms]. The maximum devices on the net may be increased if the devices
represent less then the UL [fractional unit load]. A number of devices are being produced which represent 1/4 or 1/8
the unit load. A maximum of 256 device could exist on the bus when each is at 1/8 the UL [96k ohms]. Repeaters
may be used to extend the net to any number of devices. RS-485 requires a 120 Ohm line impedance terminations
at both ends of the line (at the Receivers).
Pull-up/Pull-down resistors (Idle-line failsafe) at one end of the RS-485 bus may be used to bring the line voltage to
a steady state (200mV) value at the end of a transmission; when all drivers are in the passive state.
No maximum bus length is given for RS-485, but ends up around 1200 meters at 200kbps or 50 meters at 10Mbps.
The speed of the system and distance between devices is determined in large amount by the inter-connecting cable
The following signalling states are defined in the standards.
When the “A” terminal of the driver is negative with respect to the “B” terminal, the line is in a binary 1 (mark or Off )
state.
When the “A” terminal of the driver is positive with respect to the “B” terminal, the line is in a binary 0 (space or On )
state.
Note: Many types of equipment uses “-“ and “+” as terminal markings. In this case the “-“ terminal is equivalent to
the “A” designation and the “+” terminal equivalent to the “B” terminal.
The 8800E/8801E is set to transmit and receive with:
9600 bps.
8 data bits.
No parity.
No handshaking.
