ICS ELECTRONICS 4809A, Instruction Manual

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2-21
2
Figure 2-12 shows the 4819A's RS-232 connection to a Watlow series 96
Temperature Controller.
1
2
3
5
:
9
4819A
57
WATLOW 96
4819A
RXD
TXD
19
21
20
GPIB
Serial
Figure 2-12 4819A RS-232 Connections to a Watlow 96
2.10.2 RS-485 Connections to a Modbus Device
The 4809A, 4829A and 4899A's serial interface provides a transmit (SD)
and a receive (RD) pair of RS-422/RS-485 signals. Because most RS-485
Modbus networks are two wire, half-duplex networks, the SD and RD sig-
nal pairs have to be jumpered together in the cable connector. The 4809A,
4829A and 4899A have to be configured for RS-485 operation when used
on a two wire RS-485 network. Use the SYST:COMM:SER:RS485 ON
command to configure the units. The ON setting causes the unit to tristate
its serial transmitter when not transmitting which free's the network so the
Modbus can respond to the message.
Two wire RS-485 networks also need termination networks to bias the lines
in the 'mark' state when neither unit is transmitting. The termination networks
are made up of a pullup, a pulldown and a load resistor. The termination
networks prevent each receiver from inputting noise when nothing is being
transmitted. Use one termination network for short cables of 200 feet or
less. For longer cables, use a termination network at each end of the cable.
Set the bias voltages to approximately 2 Vdc and 2.5 Vdc. Use resistors
with an approximate value of 500 ohms/volt.
Figure 2-13 shows a method of making the RS-485 connection to a 4899A or
4809A. Figure 2-13 uses the 4899A/4809A's internal pullup resistor avail-
able at pin 20 to pullup the SD(B) and RD(B) lines. A 1 kohm resistor from
SD(A) and RD(A) to ground is the pulldown resistor. The 220 ohm load
resistor completes the circuit. The 4829A contains an internal termination
network on the RD signal pair and does not need external resistors.