This document is a description of Hot Swapping on the SCSI bus as it pertains to Paralan's SCSI Expander products. Hot Swapping is when a device or bank of devices is unplugged from the SCSI system, or connected to the system, while the system is still on. Hot Swapping becomes an issue when a system cannot afford to be shut down to add or remove a device or devices on the SCSI bus. Paralan's SCSI Converters and SCSI RegeneratoRs are designed to support Hot Swapping.
The following are Hot Swapping concerns that were accounted for when Paralan's products were designed:
Paralan's SCSI Expanders use a circuit that limits the amount of current that can be drawn through the 5-Volt termination power line. This protects the board from shorts that could happen if the 5-Volt term power line is grounded or connected to excessive current for any reason. A common way that this line gets grounded is when a single ended device gets connected with a differential (HVD) device. The 5-Volt line could get connected directly to ground. Another way is if the SCSI cable is connected upside down, which can happen with narrow SCSI internal connectors that are not keyed, or with cables that are improperly wired. Also, incidental contact with ground may happen when attempting to connect cables while power is on. Recently, improperly wired SCSI cables have become a larger problem as narrow and wide devices are being used on the same SCSI bus. This usually happens after the system has been installed and a new device is added. There are a number of sources for narrow-to-wide SCSI cables that are either not consistent in wiring the connectors or have the cables wired to work in single ended applications but cause problems when used in differential (HVD) applications.
Most manufacturers of SCSI devices use a fuse to protect their devices from the above problems. This method does keep the rest of the unit from getting hurt when the 5-Volt line is subjected to too much current draw, but once the fuse is blown, if 5-Volt power is required from the unit for proper system operation, it will not be there until the fuse is replaced. The fuse that is commonly used does not show that it is blown, other than by measurement. This has caused many problems in the field, because the only symptoms of a blown fuse can be marginal operation of the system, which could be caused by many other problems. The 5-Volt termination power fuse is in many cases not even included in the list of potential causes to look for.
Some vendors use thermal switches (circuit breakers) that will disconnect the 5 Volts when the temperature of the switch gets too hot. Paralan did not go with this method. Until the temperature of the switch is hot enough to switch off, the board could be subjected to excessive current draw and damaged. Depending on the environment that the system is in, ambient temperature could affect the length of time that the circuit is subjected to excessive current draw before the switch gets hot enough to shut off the current flow. Paralan's current limiting method is a little more expensive than the fuse or thermal switch, but it shuts off excessive current draw without allowing any of it to get through to the rest of the system, and immediately resumes operation once the cabling and devices are properly connected.
It is not supposed to happen, but it is possible to be trying to connect two devices together that are powered from sources with different ground potentials. It is possible to have the ground loop caused by such a problem produce marginal SCSI system operation. All Paralan SCSI Converters and SCSI RegeneratoRs are capacitor-coupled with power such that connecting a Paralan device to a device with a different ground potential will not affect the operation of the Paralan device. This is one of the harder problems to diagnose because of the requirements that many facilities have for UPS systems to eliminate such occurrences. Improperly installed UPS systems, however, may actually cause ground loops. It is usually the case that ground loop problems will not occur until devices are added that connect over longer SCSI cabling to devices using a different source for power.
In most cases Paralan's products are installed in static-free environments. Paralan's SCSI Expanders are designed to have any CMOS parts buffered from outside contact in order to minimize the effects of static discharge. The enclosed versions are also shielded to continue to operate when hit with electrostatic discharge and have passed the electrostatic discharge requirements for the CE Mark.
Single-ended and LVD SCSI devices will not work with differential (HVD) SCSI devices unless some type of conversion is made. They can be physically cabled together which, if not properly handled, can potentially damage devices, especially when hot swapping. Single-ended devices should not be damaged if this were to occur, however, differential (HVD) drivers could be driving into ground on half of the lines used by the differential (HVD) device. The Differential Sense (DIFFSENSE) line is used by most differential (HVD) devices to shut off the differential drivers if a single-ended device is connected to a differential (HVD) device. Paralan's SCSI Expanders shut off communications in both directions if the DIFFSENSE line is not properly handled by the connecting device. This allows operation to continue between devices on the other side even while the offending device is connected. The 5-Volt termination power line(s) also have protection as described in section 1 of this paper.