Amplicon USB232U, Instruction Manual

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USB232U/485Ui
Page 18
In a multi-drop system, the terminator resistor is only required at the device receiver located at
the far end of the cable. If this is in half duplex operation, then both ends of the bus cable are
equipped with receivers (transceivers) so termination is necessary at both ends. RS485
transmitter circuits are capable of driving a minimum load resistance of 60
Ω
, so no more than
two terminator resistors should be connected to any one bus.
4.2.3.5 RS422/485 Data Rates and Cable lengths
RS422 and RS485 standards specify data signalling rates up to 10 MBaud, with decreasing
cable length maxima at the higher rates. At a signalling rate of 1.5 MBaud by the USB485Ui,
the maximum cable length is specified as 80 metres.
Typical maximum transmission rates versus cable lengths are shown in the following table.
The data rates applicable to the PCI248H(i)/PCI249H(i) are shown in bold type.
Transmission Rate Maximum Distance
Up to 100 kBd 1200 m
115.2 kBd 1000 m
1000 kBd 100 m
10000 kBd 10 m
4.2.3.6 Cabling of RS422/485 Bus
The RS422/485 employs a differential method of signal transmission, and each bus cable has
to be a wire pair, preferably twisted and screened to keep induced and radiated noise to a
minimum. The bus distribution cable is effectively a transmission line, and appropriate
techniques should be used for installation of the cables. In a multi-drop environment, the cable
should be 'looped through' each device, or if a spur is necessary, the spur length should be
kept to a minimum.
If screened cable is used, some thought must be given to the connection of the screen (shield).
This screen should not normally be used as a ground return for non-isolated devices and it is
safest to only connect the screen at a single point to the ground of one device.
The voltage between the grounds of the various devices must not cause the common mode
voltage rating of any device on the bus to be exceeded. The USB485Ui must be used and the
allowable common mode voltage range is considerably increased.
4.2.4 USB Application Notes
4.2.4.1 Data Rates and Flow Control for USB to RS232
With USB devices data is transferred in packets. If data is to be sent from the PC, a packet is
built up by the device driver and sent to the USB scheduler. This scheduler puts a request onto
the list of tasks for the USB host controller to perform. The scheduler send these packets of
data to the USB devices every USB frame period. The ' USB Frame' (the frame period is 1
millisecond).
Data is received from USB to the PC by a polling method. The driver requests a certain
amount of data from the USB scheduler. This will be transferred in multiples of 64 bytes, which
is the maximum 'bulk packet size' on USB. The host controller reads data from the device until