MagTek SWIPE READERS WITH RS-232 - Manual (Page 9)

Section 1. Features and Specifications
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† only required to drive a 3k Ω load at 5V. This is a current of merely 5V/3kΩ=1.67mA per line. All
“port-powered” readers fundamentally require more current than 1.67mA (consider that at least
1.67mA must be supplied to a 3k
Ω 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 (typically a capacitor). In practice, however most ports can supply 8 mA at ±5V on
a single line. Some may not, and this is the reason for including a current consumption
specification for a “port-powered” device. Also note that any capacitance on the RXD (host-
referred) line will increase power consumption over the figures given here, in accordance with the
baud rate. 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 take care to avoid contention between RTS and DTR.