Powering Devices
from RS232 Ports

When powering devices from a RS232 port one needs to be careful about the amount of current available on the port. The missing component in the V.24 and V.35 specifications is the required current available on the output pins. The answer is not really one of what the specification could state but rather one which the manufacturers of the RS232 drivers will claim. Before we answer this let us view the specification a little differently.

Instead of looking for the driving currents look at what current they should be able to deliver. RS232 drivers are required to drive a 3k-ohm load to full voltage, the maximum being 15 volts which translates, using Ohm's law, into 5mA. What is also stated is that the load may consist of up to 470pF capacitance, which at 115kB (57.5Hz) is approximately 5.9k-ohm impedance. Parallelled with the 3k-ohm resistive load the load now becomes approximately 2k-ohm which translates into 7.5mA drive capability required at ±15V swing.

Industry has accepted that ±12V is the accepted RS232 voltage levels and therefore our drive capability drops to 6mA. Strangely, this is the very figure most datasheets of 1488 devices (the most common driver used) quote as the minimum drive available. The typical drive is quoted at about 10mA with some manufacturers claiming a possible maximum of 18mA, but this is not what has been found in practice. During design tests most outputs have personally been found to be between 10 and 12mA.

Back to the question - how much current can be drawn from each pin? It is believed the best approach is to keep the current drawn to the equivalent of what 3k-ohm would take at 12V being 4mA. Beyond this could deem the RS232 port unreliable. Outputs could be combined with diodes to increase the current available but caution should be exercised as it cannot be guaranteed all pins will be in the state required.

There is one exception being laptop PCs. These are often fitted with drivers that only swing ±5volts in order to minimise current drawn on the batteries.