MagTek PORT POWERED - Manual (Page 11)

Section 1. Features and Specifications
5
MECHANICAL
Dimensions
Length
Width
Height
Without bezel With Flat-faced Bezel With Extended Bezel
4.4" (111.8 mm) 4.58" (116.3 mm) 5.09" (129.3 mm)
3.51" (89.2 mm) 4.00" (101.6 mm) 4.00" (101.6 mm)
1.24" (31.5 mm) 3.00" (76.2 mm) 3.00" (76.2 mm)
Bezel Thickness Flat Faced: 0.31" (7.9mm); Extended: 0.82" (20.8 mm)
Weight Without bezel With Flat-faced Bezel With Extended Bezel
2.25 oz (65 gr) 3.85 oz (109 gr) 4.02 oz (114 gr)
* The 3.7 mA figure is for continuous data transmission at 33% duty cycle while
reading a card with both heads at once (stripe on each side). 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
both heads, with cable capacitance ‘C’ is approximately:
I
T
= (3.5 mA) + (10 V) * 33% *(9600 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
)
Subtract 1 mA from I
T
if it is known that both heads will not be used simultaneously (this
is guaranteed on single-head versions).
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.