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QTERM-IV/P40 User's Manual
Beijer Electronics Fax 801-466-8792 Web www.beijerelectronicsinc.com Phone 801-466-8770
between the + and - network lines cannot float enough to
produce a phantom start bit at the QTERM or the QTERM
will think it is receiving data when it is not. This will cause
network communications faults and other problems which
can be difficult to diagnose.
To prevent these problems, you must bias the network
towards the “marking” state so that it will always enter the
“marking” state when it is not being actively driven. Since
both the EIA-422 and -485 networks are non-inverted, the
simplest way to implement the needed bias is to “pull up”
the “+” line by connecting a 1k
Ω
to 100k
Ω
resistor
between the “+” line and power, and “pull down” the “-”
line by connecting a similar resistor between the “-” line
and ground. This will ensure that the network is always
“marking” when idle. On an EIA-422, multidrop network,
the QTERM's receive lines (“+” and “-”) are always driven
by the host so you don't need to pull these lines up or down.
The QTERM's transmit lines are sometimes tri-stated so
these must be biased to prevent the host from seeing phan-
tom noise at its receiver. The “marking” bias usually only
needs to be applied at one point on the network.
If your network is very long (over 1000 feet or so) you may
need to “terminate” the network; most networks will not
require termination. This means installing an impedance
matching resistor between the “+” and “-” lines on your
network. On an EIA-422 network, you must “terminate”
the transmit and the receive pairs separately, if you need to
terminate the network at all. A typical matching-resistor
value would be 147
Ω
, 1/4 watt. This resistor should be
installed in the QTERM farthest from the host.
4.4.2
Multidrop Data Packets
All multidrop packets are formatted as follows:
<ADR1> <ADR2> <DATA...> <EOP>
where:
<ADR1>=Unit address MSD, ASCII decimal digit, 0 to 9
<ADR2>=Unit address LSD, ASCII decimal digit, 0 to 9
<DATA....>=Data bytes, zero or more data bytes
<EOP>=End-Of-Packet Terminator byte
Unit Address. Each QTERM attached to a multidrop net-
work must have a unique address from “01” to “99”. This
address can be set either during power-on setup or by a
QDATA download. To send a packet to a QTERM, the host
puts the unit's address in the ADR1 and ADR2 fields in the
data packet. The response packet returned by the QTERM
will also contain that unit's address in these fields.
Address “00” is used to indicate a broadcast packet. Broad-
cast packets are received and processed by all QTERMs
connected to the network, regardless of their addresses.
Since all the terminals on a network process broadcast
packets at the same time, the QTERM never transmits a
response packet in answer to a broadcast packet.
Data Bytes. The <DATA....> portion of the packet consists
of zero or more bytes of data (i.e. an empty packet is valid).
In packets sent by the host, the DATA portion will usually
consist of commands and/or display data. In packets
returned by the QTERM-, the DATA portion will always
contain one of the following:
•
an ASCII ACK or NAK for acknowledgment (byte
values x06 and x15, respectively)
•
a status byte
•
a command response string
•
the contents of the QTERM's EDIT buffer
•
nothing (Null packet)
The only invalid value for a data byte (whether to or from
the host) is the EOP Terminator byte.
End-of-Packet. The EOP character is used to terminate all
data packets.
4.4.3
Receiving Data from the Host
Data received from the host is processed the same as in
Block Mode.
4.4.4
Response Packets
Response packets are generated similarly to Block Mode.
All response packets are prefixed with the unit address of
the responding terminal. ACK and NAK response packets
can be disabled with the QDATA file's [MD/Block
response mode] field.
4.4.5
Transmitting Data to the Host and the Opera-
tion of EDIT Mode
This is also similar to Block Mode with one major differ-
ence: the EDIT Buffer is not sent when the QTERM
encounters the EOP Terminator, within a string definition,
while in EDIT mode. Instead the QTERM enters PEND-
ING mode,
the LCD flashes and the keypad is disabled. The LCD flash
rate is controlled by the “Set LED Blink Rate” command. If
the flash rate is set too fast, the display will be difficult to
read.
