MagTek HALF CARD PORT - Manual (Page 14)

Half Card Port Powered Insertion Reader
6
ENVIRONMENTAL
Printed Circuit Assembly
21067502 (discontinued)
Printed Circuit Assembly
21067505 (new unit)
Temperature
Operating
Storage
4
o
F to 158
o
F (-20
o
C to 70
o
C)
-40
o
F to 176
o
F (-40
o
C to 80
o
C)
-40°F to 158°F (-40°C to 70°C)
-40
o
F to 176
o
F (-40
o
C to 80
o
C)
Humidity
Operating
Storage
10% to 90% noncondensing
10% to 90% noncondensing
* The 2.7 mA figure is for continuous data transmission at 33% duty cycle while
reading a card. Typical capacitance from TXD is about 1000 pF for our standard 2 m
cable. Minimum DTR current ‘I
T
’ required for continuous transmission at 33% duty
cycle, while reading a card, with cable capacitance ‘C’ and arbitrary baud rate is
approximately:
I
T
= (2.5 mA) + (10 V) * 33% *(baud rate in Hz) / 2 * C.
Maximum transmission burst time ‘T’ at 33% duty cycle for RS-232 compatibility is
approximately:
T = (64 μF) * (5 V - 3.4 V) / (I
T
- I
S
), where I
S
is the current supplied by the DTR
line (T is unlimited for I
S
> I
T
)
A note about “port-powered” readers: These readers operate off some combination of otherwise
unused RS-232 lines, DTR and TXD from the host in this case. Per the RS-232 specification,
these lines are only required to drive a 3 kΩ load at +/-5 V. This is a current of merely 5 V / 3
kΩ=1.67 mA per line. All “port-powered” readers fundamentally require more current than 1.67
mA (consider that at least 1.67 mA must be supplied to a 3 kΩ load, and some extra current is
needed for the circuit that does so). Thus these readers are not technically guaranteed to work
unless multiple unused lines are used for power and/or some duty cycle limit is imposed on
transmitting while employing an energy storage device (a capacitor). In practice, however most
ports can easily supply the 2.7 mA at +5 V required by this new reader on DTR and the near-
specification average TXD (from host) current at -5 V. This new reader is MagTek’s lowest
current “port-powered” reader to date. Strictly speaking, some RS-232 ports may not supply the
required current, and this is the reason for including a current consumption specification for a
“port-powered” device. The current drive capability of an RS-232 port is not typically specified,
so experimentation may be required in a particular application. If more current is needed for the
positive supply, RTS may be paralleled with DTR (both host-referenced) in the cabling to the
unit. If this is done, the host must of course hold RTS high.